op.c:leave_scope: use mg_free before sv_force_normal

This is part of ticket #119295.

Commit 7fa949d (5.19.3) allowed copy-on-write with compile-time con-
stants.  That caused this to fail:

use Variable::OnDestruct:
{
 my $h = "foo";
 on_destruct $h, sub { warn defined $_[0] ? $_[0] : "undef" };
 $x++; # prev statement must not be last in the block
}

It prints undef instead of foo.

It turns out this is much older:

use Variable::OnDestruct;
{
 my $h = __PACKAGE__;
 on_destruct $h, sub { warn defined $_[0] ? $_[0] : "undef" };
 $x++; # prev statement must not be last in the block
}

This one prints undef starting with 5.17.3 (a6d7a4ac1).

But even before that, this prints undef:

use Variable::OnDestruct;
{
 my $h = \1;
 on_destruct $h, sub { warn defined $_[0] ? $_[0] : "undef" };
 $x++; # prev statement must not be last in the block
}

In all these cases, the scalar is getting undefined before free magic
triggers (Variable::OnDestruct uses free magic).

Usually when a scalar is freed, the magic is triggered before anything
else.  When a lexical scalar is ‘freed’ on scope exit (cleared for
reuse on scope entry), the order is different.  References, globs and
copy-on-write scalars become undefined (via sv_force_normal) before
magic is triggered.

There is no reason for the order to be different here, and it causes
unpredictable behaviour (you never know when you will or will not have
a cow).  So change the order in scope exit to match regular freeing.