blockchain: Fix bogus error message.
blockchain: Fix bogus error message.

In the presence of overflow, the actual output sum is still not
negative and should not be reported as so.

Stash-Crypto/btcd,dan-da/btcd,ppcsuite/ppcd,jcmincke/dcrd,jongillham/btcd,ppcsuite/ppcd,stevenroose/btcd,dan-da/btcd,adiabat/btcd,Stash-Crypto/btcd,cqswxj2013/btcd,davecgh/btcd,jrick/dcrd,tb00/btcd,ltcsuite/ltcd,jrick/btcd,wallclockbuilder/btcd,stevenroose/btcd,runeaune/btcd,decred/dcrd,dajohi/btcd,jcmincke/dcrd,btcsuite/btcd,wallclockbuilder/btcd,BitGo/prova,aakselrod/btcd,soapboxsys/ombfullnode,dajohi/btcd,jrick/dcrd,ltcsuite/ltcd,aakselrod/btcd,adiabat/btcd,jrick/btcd,Roasbeef/btcd,hectorj/btcd,jcmincke/dcrd,soapboxsys/ombfullnode,tb00/btcd,jrick/btcd,dajohi/btcd,decred/dcrd,cqswxj2013/btcd,hectorj/btcd,davecgh/btcd,btcsuite/btcd,Roasbeef/btcd,runeaune/btcd,jongillham/btcd,BitGo/prova