BUG/MINOR: pattern: remove useless allocation of unused trash in pat_parse_reg()

Just like previous patch, this is a remains of an early implementation. Also
fix the outdated comments above. The fix may be backported to 1.5 though the
bug cannot be triggerred, thus it's just a matter of keeping the code clean.
(cherry picked from commit 5def8ef7864339b3e1e01a6c621e0c831c9e3bac)
1 file changed