MFC r300378:

MFC r300378:
libc/regex: fix two buffer underruns.

Fix some rather complex regex issues found on OpenBSD as part of some
ongoing work to fix a sed(1) bug.

Curiously the OpenBSD tests don't trigger segfaults on FreeBSD but the
bugs were confirmed by running a port of FreeBSD's regex under OpenBSD's
malloc. Huge thanks to Ingo for confirming the behavior.

Obtained from:	OpenBSD (CVS 1.20, 1.21)

git-svn-id: svn://svn.freebsd.org/base/stable/9@300964 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f