assertion - ensure that the hek argument to Perl_unshare_hek() is not
NULL, because if it is, then S_unshare_hek_or_pvn() will merrily
unshare the string "" instead.
p4raw-id: //depot/perl@30440
void
Perl_unshare_hek(pTHX_ HEK *hek)
{
+ assert(hek);
unshare_hek_or_pvn(hek, NULL, 0, 0);
}