Prevent empty transactions from being added to vtxPrev
Prevent empty transactions from being added to vtxPrev

CWalletTx::AddSupportingTransactions() was adding empty transaction
to vtxPrev in some cases. Skip over these.

Part one of the solution to #3190. This prevents invalid vtxPrev from
entering the wallet, but not current ones being transmitted.

CrimeaCoin/crimeacoin,Ziftr/litecoin,Electronic-Gulden-Foundation/egulden,wekuiz/wekoin,kryptokredyt/ProjektZespolowyCoin,vertcoin/eyeglass,DrCrypto/darkcoin,oklink-dev/litecoin_block,plankton12345/litecoin,netswift/vertcoin,shapiroisme/datadollar,jiffe/cosinecoin,dakk/soundcoin,Dajackal/Ronpaulcoin,zzkt/solarcoin,greencoin-dev/greencoin-dev,tensaix2j/bananacoin,marscoin/marscoin,netswift/vertcoin,degenorate/Deftcoin,brightcoin/brightcoin,novaexchange/EAC,zzkt/solarcoin,mooncoin-project/mooncoin-landann,DrCrypto/darkcoin,Cancercoin/Cancercoin,chrisfranko/aiden,cryptcoins/cryptcoin,ccoin-project/ccoin,scamcoinz/scamcoin,marscoin/marscoin,novaexchange/EAC,koharjidan/litecoin,icook/vertcoin,netswift/vertcoin,icook/vertcoin,razor-coin/razor,IlfirinIlfirin/shavercoin,Rav3nPL/doubloons-08,privatecoin/privatecoin,greencoin-dev/greencoin-dev,Dajackal/Ronpaulcoin,mrtexaznl/mediterraneancoin,Dajackal/Ronpaulcoin,ohac/sakuracoin,coinkeeper/2015-06-22_19-13_florincoin,tensaix2j/bananacoin,novaexchange/EAC,earthcoinproject/earthcoin,bootycoin-project/bootycoin,Checkcoin/checkcoin,ccoin-project/ccoin,IlfirinCano/shavercoin,plankton12345/litecoin,coinkeeper/2015-06-22_18-46_razor,zzkt/solarcoin,shadowoneau/ozcoin,Cancercoin/Cancercoin,brightcoin/brightcoin,dakk/soundcoin,Tetcoin/tetcoin,brishtiteveja/sherlockholmescoin,bitgoldcoin-project/bitgoldcoin,mooncoin-project/mooncoin-landann,scamcoinz/scamcoin,Electronic-Gulden-Foundation/egulden,ohac/sakuracoin,coinkeeper/2015-06-22_18-51_vertcoin,ANCompany/birdcoin-dev,ccoin-project/ccoin,icook/vertcoin,CrimeaCoin/crimeacoin,Ziftr/litecoin,coinkeeper/2015-06-22_19-13_florincoin,ohac/sha1coin,shurcoin/shurcoin,micryon/GPUcoin,KaSt/equikoin,neutrinofoundation/neutrino-digital-currency,Coinfigli/coinfigli,coinkeeper/2015-06-22_18-51_vertcoin,zemrys/vertcoin,shadowoneau/ozcoin,Electronic-Gulden-Foundation/egulden,plankton12345/litecoin,united-scrypt-coin-project/unitedscryptcoin,zzkt/solarcoin,alexandrcoin/vertcoin,ohac/sakuracoin,Litecoindark/LTCD,razor-coin/razor,okinc/litecoin,wekuiz/wekoin,zemrys/vertcoin,Tetcoin/tetcoin,cryptcoins/cryptcoin,shadowoneau/ozcoin,dakk/soundcoin,IlfirinCano/shavercoin,netswift/vertcoin,Litecoindark/LTCD,Checkcoin/checkcoin,bitgoldcoin-project/bitgoldcoin,bitgoldcoin-project/bitgoldcoin,Rav3nPL/doubloons-08,vertcoin/eyeglass,plankton12345/litecoin,cryptcoins/cryptcoin,razor-coin/razor,shurcoin/shurcoin,richo/dongcoin,shurcoin/shurcoin,degenorate/Deftcoin,coinkeeper/2015-06-22_18-46_razor,stamhe/litecoin,coinkeeper/2015-06-22_18-51_vertcoin,netswift/vertcoin,novaexchange/EAC,KaSt/equikoin,koharjidan/litecoin,stamhe/litecoin,icook/vertcoin,coinkeeper/2015-06-22_18-46_razor,Rav3nPL/doubloons-08,coinkeeper/2015-06-22_18-51_vertcoin,tensaix2j/bananacoin,kryptokredyt/ProjektZespolowyCoin,richo/dongcoin,Ziftr/litecoin,united-scrypt-coin-project/unitedscryptcoin,mooncoin-project/mooncoin-landann,koharjidan/litecoin,razor-coin/razor,coinkeeper/2015-06-22_19-13_florincoin,cryptcoins/cryptcoin,gwangjin2/gwangcoin-core,scamcoinz/scamcoin,novaexchange/EAC,degenorate/Deftcoin,coinkeeper/2015-06-22_18-46_razor,KaSt/equikoin,micryon/GPUcoin,Tetcoin/tetcoin,micryon/GPUcoin,Rav3nPL/doubloons-08,tensaix2j/bananacoin,cryptcoins/cryptcoin,neutrinofoundation/neutrino-digital-currency,coinkeeper/2015-06-22_19-13_florincoin,mrtexaznl/mediterraneancoin,Czarcoin/czarcoin,bitgoldcoin-project/bitgoldcoin,marscoin/marscoin,shapiroisme/datadollar,ANCompany/birdcoin-dev,degenorate/Deftcoin,mooncoin-project/mooncoin-landann,koharjidan/litecoin,mrtexaznl/mediterraneancoin,privatecoin/privatecoin,koharjidan/litecoin,okinc/litecoin,greencoin-dev/greencoin-dev,Czarcoin/czarcoin,wekuiz/wekoin,CrimeaCoin/crimeacoin,oklink-dev/litecoin_block,coinkeeper/2015-04-19_21-20_litecoindark,zemrys/vertcoin,shapiroisme/datadollar,ronpaulcoin/ronpaulcoin,gwangjin2/gwangcoin-core,bootycoin-project/bootycoin,shadowoneau/ozcoin,ohac/sha1coin,richo/dongcoin,DrCrypto/darkcoin,netswift/vertcoin,richo/dongcoin,brishtiteveja/sherlockholmescoin,ronpaulcoin/ronpaulcoin,zemrys/vertcoin,oklink-dev/litecoin_block,oklink-dev/litecoin_block,wekuiz/wekoin,Coinfigli/coinfigli,CrimeaCoin/crimeacoin,icook/vertcoin,ANCompany/birdcoin-dev,brightcoin/brightcoin,brishtiteveja/sherlockholmescoin,mooncoin-project/mooncoin-landann,ccoin-project/ccoin,shapiroisme/datadollar,shadowoneau/ozcoin,privatecoin/privatecoin,mrtexaznl/mediterraneancoin,mrtexaznl/mediterraneancoin,IlfirinCano/shavercoin,vertcoin/eyeglass,shapiroisme/datadollar,okinc/litecoin,Cancercoin/Cancercoin,alexandrcoin/vertcoin,neutrinofoundation/neutrino-digital-currency,Electronic-Gulden-Foundation/egulden,Litecoindark/LTCD,zebrains/Blotter,marscoin/marscoin,dakk/soundcoin,IlfirinCano/shavercoin,IlfirinIlfirin/shavercoin,Ziftr/litecoin,coinkeeper/2015-04-19_21-20_litecoindark,ohac/sha1coin,Checkcoin/checkcoin,ohac/sakuracoin,stamhe/litecoin,chrisfranko/aiden,coinkeeper/2015-04-19_21-20_litecoindark,ohac/sha1coin,IlfirinCano/shavercoin,scamcoinz/scamcoin,Czarcoin/czarcoin,earthcoinproject/earthcoin,scamcoinz/scamcoin,alexandrcoin/vertcoin,ANCompany/birdcoin-dev,wekuiz/wekoin,shurcoin/shurcoin,greencoin-dev/greencoin-dev,ohac/sha1coin,jiffe/cosinecoin,bootycoin-project/bootycoin,razor-coin/razor,Coinfigli/coinfigli,zebrains/Blotter,zemrys/vertcoin,neutrinofoundation/neutrino-digital-currency,ronpaulcoin/ronpaulcoin,coinkeeper/2015-06-22_19-13_florincoin,Checkcoin/checkcoin,micryon/GPUcoin,Electronic-Gulden-Foundation/egulden,brishtiteveja/sherlockholmescoin,coinkeeper/2015-04-19_21-20_litecoindark,Cancercoin/Cancercoin,dakk/soundcoin,privatecoin/privatecoin,brightcoin/brightcoin,marscoin/marscoin,zemrys/vertcoin,kryptokredyt/ProjektZespolowyCoin,Checkcoin/checkcoin,zebrains/Blotter,ronpaulcoin/ronpaulcoin,jiffe/cosinecoin,Tetcoin/tetcoin,chrisfranko/aiden,bitgoldcoin-project/bitgoldcoin,zebrains/Blotter,Litecoindark/LTCD,kryptokredyt/ProjektZespolowyCoin,zebrains/Blotter,united-scrypt-coin-project/unitedscryptcoin,earthcoinproject/earthcoin,Cancercoin/Cancercoin,Tetcoin/tetcoin,IlfirinIlfirin/shavercoin,IlfirinIlfirin/shavercoin,earthcoinproject/earthcoin,kryptokredyt/ProjektZespolowyCoin,okinc/litecoin,vertcoin/eyeglass,Czarcoin/czarcoin,micryon/GPUcoin,Rav3nPL/doubloons-08,ronpaulcoin/ronpaulcoin,brishtiteveja/sherlockholmescoin,ohac/sakuracoin,greencoin-dev/greencoin-dev,koharjidan/litecoin,KaSt/equikoin,Ziftr/litecoin,Dajackal/Ronpaulcoin,vertcoin/eyeglass,chrisfranko/aiden,tensaix2j/bananacoin,coinkeeper/2015-06-22_18-46_razor,KaSt/equikoin,bootycoin-project/bootycoin,Electronic-Gulden-Foundation/egulden,CrimeaCoin/crimeacoin,marscoin/marscoin,gwangjin2/gwangcoin-core,brightcoin/brightcoin,DrCrypto/darkcoin,plankton12345/litecoin,gwangjin2/gwangcoin-core,bootycoin-project/bootycoin,richo/dongcoin,Czarcoin/czarcoin,ANCompany/birdcoin-dev,okinc/litecoin,gwangjin2/gwangcoin-core,chrisfranko/aiden,zzkt/solarcoin,privatecoin/privatecoin,jiffe/cosinecoin,united-scrypt-coin-project/unitedscryptcoin,oklink-dev/litecoin_block,degenorate/Deftcoin,Dajackal/Ronpaulcoin,shurcoin/shurcoin,united-scrypt-coin-project/unitedscryptcoin,earthcoinproject/earthcoin,Coinfigli/coinfigli,coinkeeper/2015-04-19_21-20_litecoindark,Coinfigli/coinfigli,Litecoindark/LTCD,coinkeeper/2015-06-22_18-51_vertcoin,IlfirinIlfirin/shavercoin,alexandrcoin/vertcoin,stamhe/litecoin