Tweak the GV downgrading of f7461760 to avoid free or nearly freed GVs.

Tweak the GV downgrading of f7461760 to avoid free or nearly freed GVs.

It's possible during global destruction that the GV is freed before the
optree. Whilst the SvREFCNT_inc is happy to bump from 0 to 1 on a freed SV,
the corresponding SvREFCNT_dec from 1 to 0 will trigger an assertion
failure, because the entry to sv_clear checks that the scalar is not already
freed.  A check of for !SvIS_FREED(gv) turns out to be invalid, because
during global destruction the reference count can be forced down to zero
(with SVf_BREAK set).  In which case raising to 1 and then dropping to 0
triggers cleanup before it should happen.  I *think* that this might
actually be a general, systematic, weakness of the whole idea of SVf_BREAK,
in that code *is* allowed to raise and lower references during global
destruction, so any *valid* code that happens to do this during global
destruction might well trigger premature cleanup.