Test reference to unavailable lexical variable

This is related to #123172.

v5.21.3-644-ge52eb89 inadvertently fixed this old bug, which was prob-
ably introduced by the jumbo closure patch:

$ perl5.8.9 -le 'sub { my $f; BEGIN { $ref = \$f; $f = 7; $$ref = 8; print $f } }'
8
$ perl5.10 -le 'sub { my $f; BEGIN { $ref = \$f; $f = 7; $$ref = 8; print $f } }'
7
$ perl5.20.1 -le 'sub { my $f; BEGIN { $ref = \$f; $f = 7; $$ref = 8; print $f } }'
7
$ perl5.21.5 -le 'sub { my $f; BEGIN { $ref = \$f; $f = 7; $$ref = 8; print $f } }'
8

\ was returning a *copy* of its referent if the latter closed over an
anonymous sub’s stale variable.