FlushBlockFile(): check for valid FILE pointer

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

src/main.cpp

@@ -1540,13 +1540,17 @@ void static FlushBlockFile()
     posOld.nPos = 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);