warnings: Compiler warning on memset usage for non-trivial type
warnings: Compiler warning on memset usage for non-trivial type

Problem:
- IS_TRIVIALLY_CONSTRUCTIBLE macro does not work correctly resulting
  in `memset()` usage to set a non-trivial type to 0 when
  `nontrivial_t` is passed in from the tests.
- Warning reported by GCC when compiling with `--enable-werror`.

Solution:
- Use the standard algorithm `std::fill_n()` and let the compiler
  determine the optimal way of looping or using `memset()`.

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quets/namecoin-core,namecoin/namecoin-core,practicalswift/bitcoin,rnicoll/bitcoin,anditto/bitcoin,domob1812/bitcoin,domob1812/namecore,n1bor/bitcoin,bitcoin/bitcoin,bespike/litecoin,vertcoin/vertcoin,achow101/bitcoin,droark/bitcoin,monacoinproject/monacoin,ahmedbodi/vertcoin,mruddy/bitcoin,EthanHeilman/bitcoin,DigitalPandacoin/pandacoin,untrustbank/litecoin,ahmedbodi/vertcoin,wellenreiter01/Feathercoin,ajtowns/bitcoin,rnicoll/dogecoin,peercoin/peercoin,jlopp/statoshi,particl/particl-core,sstone/bitcoin,rnicoll/bitcoin,lateminer/bitcoin,DigitalPandacoin/pandacoin,pstratem/bitcoin,Sjors/bitcoin,afk11/bitcoin,OmniLayer/omnicore,myriadcoin/myriadcoin,CryptArc/bitcoin,wellenreiter01/Feathercoin,droark/bitcoin,prusnak/bitcoin,sipsorcery/bitcoin,achow101/bitcoin,cdecker/bitcoin,kallewoof/bitcoin,tecnovert/particl-core,dscotese/bitcoin,litecoin-project/litecoin,peercoin/peercoin,namecoin/namecore,wellenreiter01/Feathercoin,monacoinproject/monacoin,mm-s/bitcoin,dscotese/bitcoin,namecoin/namecoin-core,jamesob/bitcoin,dscotese/bitcoin,pstratem/bitcoin,yenliangl/bitcoin,EthanHeilman/bitcoin,domob1812/namecore,anditto/bitcoin,pataquets/namecoin-core,pstratem/bitcoin,cdecker/bitcoin,DigitalPandacoin/pandacoin,mitchellcash/bitcoin,litecoin-project/litecoin,sstone/bitcoin,particl/particl-core,r8921039/bitcoin,litecoin-project/litecoin,cdecker/bitcoin,MeshCollider/bitcoin,ElementsProject/elements,cdecker/bitcoin,myriadteam/myriadcoin,ajtowns/bitcoin,rnicoll/bitcoin,droark/bitcoin,JeremyRubin/bitcoin,practicalswift/bitcoin,bespike/litecoin,sipsorcery/bitcoin,droark/bitcoin,jnewbery/bitcoin,OmniLayer/omnicore,prusnak/bitcoin,myriadteam/myriadcoin,ajtowns/bitcoin,mitchellcash/bitcoin,mruddy/bitcoin,untrustbank/litecoin,tecnovert/particl-core,jnewbery/bitcoin,nikkitan/bitcoin,fujicoin/fujicoin,midnightmagic/bitcoin,GroestlCoin/GroestlCoin,OmniLayer/omnicore,jonasschnelli/bitcoin,apoelstra/bitcoin,bitcoinsSG/bitcoin,myriadteam/myriadcoin,afk11/bitcoin,wellenreiter01/Feathercoin,mruddy/bitcoin,monacoinproject/monacoin,MeshCollider/bitcoin,n1bor/bitcoin,OmniLayer/omnicore,MarcoFalke/bitcoin,particl/particl-core,sstone/bitcoin,pstratem/bitcoin,tjps/bitcoin,bitcoinknots/bitcoin,DigitalPandacoin/pandacoin,fanquake/bitcoin,gjhiggins/vcoincore,sstone/bitcoin,dscotese/bitcoin,EthanHeilman/bitcoin,DigitalPandacoin/pandacoin,tecnovert/particl-core,afk11/bitcoin,fanquake/bitcoin,FeatherCoin/Feathercoin,jtimon/bitcoin,domob1812/namecore,JeremyRubin/bitcoin,untrustbank/litecoin,practicalswift/bitcoin,jtimon/bitcoin,prusnak/bitcoin,anditto/bitcoin,myriadcoin/myriadcoin,ajtowns/bitcoin,achow101/bitcoin,EthanHeilman/bitcoin,GroestlCoin/GroestlCoin,rnicoll/bitcoin,bitcoinsSG/bitcoin,sstone/bitcoin,mruddy/bitcoin,ahmedbodi/vertcoin,MeshCollider/bitcoin,n1bor/bitcoin,vertcoin/vertcoin,GroestlCoin/bitcoin,sipsorcery/bitcoin,myriadcoin/myriadcoin,jlopp/statoshi,FeatherCoin/Feathercoin,Sjors/bitcoin,CryptArc/bitcoin,r8921039/bitcoin,myriadcoin/myriadcoin,kallewoof/bitcoin,namecoin/namecore,gjhiggins/vcoincore,untrustbank/litecoin,untrustbank/litecoin,jonasschnelli/bitcoin,AkioNak/bitcoin,jtimon/bitcoin