Fix assert fail when fetching pos clobbers ref with undef

Fix assert fail when fetching pos clobbers ref with undef

pos($x) returns a special magical scalar that sets the match position
on $x.  Calling pos($x) twice will provide two such scalars.  If we
set one of them to a reference, set the other to undef, and then read
the first, all hail breaks loose, because of the use of SvOK_off.

SvOK_off is not sufficient if arbitrary values can be assigned by Perl
code.  Globs, refs and regexps (among others) need special handling,
which sv_setsv knows how to do.