simplify the return code in (??{})

simplify the return code in (??{})

In the case where (??{...}) returns a non-reference with SMG,
there are two branches: if it has GMG too, we assert it hasn't got
PERL_MAGIC_qr, thus (intentionally) making redundant the following
sv_unmagic(ret, PERL_MAGIC_qr); otherwise we mg_find(ret, PERL_MAGIC_qr)
it.

This can all be reduced to a single mg_find() with no change in code
logic.