op.c: Temporary fix for bad read

We can’t read *name at this point, because the
SAVEFREEOP(o)/LEAVE_SCOPE above will have freed the SV owning the
string buffer.

We can’t do pad_add_weakref before LEAVE_SCOPE, because it will add
the entry to the wrong pad.

This fix is only temporary, because one more behaviour change (ena-
bling read-only ops for special blocks other than BEGIN) will allow be
to simplify it.