Merge pull request #2057 from Diapolo/FlushBlockFile

FlushBlockFile(): check for valid FILE pointer

src/main.cpp

@@ -1538,12 +1538,16 @@ void static FlushBlockFile()
     CDiskBlockPos posOld(nLastBlockFile, 0);
 
     FILE *fileOld = OpenBlockFile(posOld);
+    if (fileOld) {
         FileCommit(fileOld);
         fclose(fileOld);
+    }
 
     fileOld = OpenUndoFile(posOld);
+    if (fileOld) {
         FileCommit(fileOld);
         fclose(fileOld);
+    }
 }
 
 bool FindUndoPos(int nFile, CDiskBlockPos &pos, unsigned int nAddSize);