regex: fix freeing g->charjump in low memory condition

computejumps() moves g->charjump to a position relativ to the value of
CHAR_MIN. As such, g->charjump doesn't necessarily point to the address
actually allocated. While regfree() takes that into account, the low
memory handling in regcomp_internal() doesn't. Fix that by free'ing
the actually allocated address, as in regfree().

MFC After: 2 weeks
Reviewed by: imp,jrtc27
Pull Request: https://github.com/freebsd/freebsd-src/pull/692