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.

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