Perl_sv_uni_display() needs to be aware of RX_WRAPPED()

Commit 8d919b0a35f2b57a changed the storage location of the string in
SVt_REGEXP. It updated most code to deal with this, but missed the use of
SvPVX_const() in Perl_sv_uni_display(). This breaks dumping regular
expressions which have the UTF-8 flag set.