Prevent SEGV on corrupted msdos directories

[sevan]

Summary

By Veo Zhang via NetBSD
Tested it builds as an individual component (not as part of build world) but
haven't performed any tests on a file system.

[bcr]

You need to find someone more knowledgeable in that area (i.e. src-committer) to look over that change. Maybe someone who recently made some change to the same code area: https://svnweb.freebsd.org/base/head/sbin/fsck_msdosfs/

[pfg]

It is not incorrect but it looks somewhat weird: Perhaps it's just an off-by-one and changing k >=0 to k > 0 in line 620 solves it more cleanly.

I'll think of it for a while :).

[sevan]

Already in.