Prevent overflow for block number in buffile.c

As coded, the start block calculated by BufFileAppend() would overflow
once more than 16k files are used with a default block size.  This issue
existed before b1e5c9fa9a, but there's no reason not to be clean about
it.

Per report from Coverity, with a fix suggested by Tom Lane.

src/backend/storage/file/buffile.c

@@ -904,7 +904,7 @@ BufFileSize(BufFile *file)
 int64
 BufFileAppend(BufFile *target, BufFile *source)
 {
-	int64		startBlock = target->numFiles * BUFFILE_SEG_SIZE;
+	int64		startBlock = (int64) target->numFiles * BUFFILE_SEG_SIZE;
 	int			newNumFiles = target->numFiles + source->numFiles;
 	int			i;