On corrupted FATfs may have invalid ->i_start.  To handle it, this checks
->i_start before using, and return proper error code.

Link: http://lkml.kernel.org/r/87o9f8y1t5.fsf_-_@mail.parknet.co.jpSigned-off-by: NOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reported-by: NAnatoly Trosinenko <anatoly.trosinenko@gmail.com>
Tested-by: NAnatoly Trosinenko <anatoly.trosinenko@gmail.com>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>