pack-mtimes: avoid closing a bogus file descriptor

In 94cd775a6c52 (pack-mtimes: support reading .mtimes files,
2022-05-20), code was added to close the file descriptor corresponding
to the mtimes file.

However, it is possible that opening that file failed, in which case we
are closing a file descriptor with the value `-1`. Let's guard that
`close()` call.

Reported by Coverity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-mtimes.c

@@ -89,7 +89,8 @@ cleanup:
 		*data_p = data;
 	}
 
-	close(fd);
+	if (fd >= 0)
+		close(fd);
 	return ret;
 }