regmatch(): silence OpenWatcom compiler warnings

regmatch(): silence OpenWatcom compiler warnings

There are two MULTICALL variables which are assigned to in one branch,
but not the other. The values of these variables are never used.
OpenWatcom complains about this, and the easiest way to shut it up may be
to assign null values in the other branch. As bulk88 pointed out, these
assignments should be optimised away anyway.

Based on a patch originally submitted by Reini Urban, but without
removing the PERL_UNUSED_VAR's required for gcc.

See the thread <50A2B8E5.5030401@gknw.net> for more info.