Set PL_statgv to null when freed or coerced

Set PL_statgv to null when freed or coerced

If PL_statgv is not set to null when freed, that same SV could be
reused for another GV, in which case -T _ will then use another handle
unrelated to the previous stat.

Similarly, if PL_statgv points to a fake glob that gets coerced into
a non-glob before it is freed, it will not follow the code path in
sv_free that sets PL_statgv to null.  Furthermore, if it becomes a GV
again, it could be a completely different filehandle, unrelated to the
previous stat.