handle weird/undef (?{}), (??{}) return value

handle weird/undef (?{}), (??{}) return value

All three code block variants: (?{}), (??{}), (?(?{}X|Y)),
make use of the return value of the block, either to set $^R, determine
truth, or to interpret as a pattern.  Evaluating this value may trigger
magic calls, uninitialized var warnings etc. Make sure that this
processing happens in the right environment; specifically, before we've
restored vars and paren indices, and we set PL_op temporarily to NULL so
that uninit var warnings don't try to look in the wrong place: neither the
outer op (eg OP_MATCH) nor the inner op (the last op of the code block:
currently happens to be OP_NULL, but that's a bug; will eventually be last
*real* op, e.g. padsv) are suitable for identifying where the warning came
from.

For the (??{}) case, if we can't extract a pre-compiled regex from it,
we force it to a PV, making a temp copy if necessary.