bsdgrep(1): Fix errors with invalid expressions

Invalid expressions with an ultimate compiled pattern length of 0 (e.g.,
"grep -E {") were not taken into account and caused a segfault while trying
to fill in the good suffix table.

Submitted by: Kyle Evans <kevans91 at ksu.edu>
Reviewed by: me
Differential Revision: https://reviews.freebsd.org/D10113