bsdgrep: fix segfault with --mmap

r313948 partially fixed --mmap behavior but was incomplete. This commit
generally reverts it and does it the more correct way- by just consuming
the rest of the buffer and moving on.

PR: 219402
Submitted by: Kyle Evans <kevans91@ksu.edu>
Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D10820