fat: fix uninitialized variable

fat: fix uninitialized variable

syszbot produced this with a corrupted fs image.  In theory, however an IO
error would trigger this also.

This affects just an error report, so should not be a serious error.

Link: https://lkml.kernel.org/r/87r08wjsnh.fsf@mail.parknet.co.jp
Link: https://lkml.kernel.org/r/66ff2c95.050a0220.49194.03e9.GAE@google.com
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reported-by: syzbot+ef0d7bc412553291aa86@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>