py/mpz: Avoid undefined behavior decrementing NULL.

In the case where an mpz number is zero, its `len` is 0 and its `dig` is
NULL.  In that case, decrementing NULL via `d--` is undefined behavior
according to the C specification.

Restructuring the loops in this way avoids undefined behavior.

Also, ensure that these cases are tested in the coverage test.  This
doesn't make much difference now, but would otherwise cause errors later
when the undefined behavior sanitizer is employed in CI.

Signed-off-by: Jeff Epler <jepler@gmail.com>

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