FlushBlockFile(): check for valid FILE pointer
FlushBlockFile(): check for valid FILE pointer

- don't call FileCommit() and fclose() if no valid FILE pointer was
  returned by OpenBlockFile()

domob1812/crowncoin,Infernoman/crowncoin,syscoin/syscoin,dobbscoin/dobbscoin-source,Infernoman/crowncoin,Earlz/dobbscoin-source,syscoin/syscoin,Earlz/dobbscoin-source,Earlz/dobbscoin-source,syscoin/syscoin,Crowndev/crowncoin,domob1812/crowncoin,domob1812/crowncoin,domob1812/crowncoin,Crowndev/crowncoin,Earlz/dobbscoin-source,syscoin/syscoin,Infernoman/crowncoin,dobbscoin/dobbscoin-source,domob1812/crowncoin,syscoin/syscoin,domob1812/crowncoin,syscoin/syscoin,Earlz/dobbscoin-source,dobbscoin/dobbscoin-source,dobbscoin/dobbscoin-source,Infernoman/crowncoin,Infernoman/crowncoin,Earlz/dobbscoin-source,syscoin/syscoin,Crowndev/crowncoin,Crowndev/crowncoin,syscoin/syscoin,Crowndev/crowncoin,Crowndev/crowncoin,dobbscoin/dobbscoin-source,dobbscoin/dobbscoin-source,Infernoman/crowncoin