Fix regexp bug I introduced back in July 2007.
Some pathological regular expressions will now
produce regexp overflow errors (safe, they just
won't execute properly) again, but common regexps
will now find the right matches when there is a
choice to be made.

This bug was causing the creation of /sys/src/9/port/systab.h
to fail.  After applying it and rebuilding libregexp, sed, 
sam, and acme, you can see that it worked by doing

	cd /sys/src/9/port; rc mksystab | grep SYSsys

and not getting any lines.  Right now you get

	cpu% cd /sys/src/9/port; rc mksystab | grep SYSsys
		[SYSR1]		SYSsysr1,
	cpu% 

Russ