Bail out of corrupt directory entries during boot

A directory with a zero-valued d_reclen causes boot to hang, This
patch checks for directory entries with value zero and bails out
rather than spin forever. The hope is that the user has other options.

Sadly this was reported on Jun 16 2015 and is just now attended to
as part of a sweep of old unclosed phabricator reports.

Reported-by: Daniel O'Connor darius-dons.net.au
Differential Revision: https://reviews.freebsd.org/D2844
MFC-after: 1 week
Sponsored-by: Netflix