[perl #70748] threads panic in del_backref

[perl #70748] threads panic in del_backref

This was caused by change 34210/41fae7a, which simply reveals a bug that
already existed.

A sub returned from a thread brings a lot of baggage with it, including
some globs. There is this comment near the top of Perl_sv_dup in the
if(param->flags & CLONEf_JOIN_IN) block that reads:
        /** don't clone stashes if they already exist **/

Then later on, under case SVt_PVGV:
        /* Don't call sv_add_backref here as it's going to be
           created as part of the magic cloning of the symbol
           table. */

So what’s happening is that there is a glob with no back-reference in its
stash, which glob is sometimes freed after the stash, so it induces the
panic.