Cloning a format whose outside has been undefined

Cloning a format whose outside has been undefined

This has crashed ever since 71f882da8, because the format tries to
close over a pad that does not exist:

sub x {
    {my ($a,$b,$c,$d,$e,$f,$g,$h,$i,$j,$k,$l,$m,$n,$o,$p,$q,$r,$s,$t,$u)}
    my $z;
    format =
@<<<
$z
.
}
undef &x;
write;

This commit adds checks for nonexistent pads, producing the ‘Variable
is not available’ warning in cases like this.

(cherry-picked from f2ead8b)