avoid leak assigning regexp to non-COW string

avoid leak assigning regexp to non-COW string

In something like

    $s = substr(.....); # $s now a non-COW SvPOK() SV
    $r = qr/..../;
    $s = $$r;

$s's previous string buffer would leak when an SVt_REGEXP type SV is
assigned to it.

Worse, if $s was an SVt_PVPV, it would fail an assert on debugging
builds.

The fix is to make sure any remaining stringy stuff is cleaned up
before copying the REGEXP.