Quad_t and Uquad_t cannot unpack as NVs.

If IVSIZE >= 8, a Quad_t is always >= IV_MIN, and <= IV_MAX, and an
Uquad_t is always (>= 0 aka UV_MIN and) <= UV_MAX; they cannot escape
their quadness and be NVs.  (This logic may fail if Quad_t is not 8
bytes, but then other things would no doubt fail.)

Also tighten the logic by adding HAS_QUAD, also for the pack case.

Fix for Coverity perl5 CID 28942.