gh-120010: Fix invalid (nan+nanj) results in _Py_c_prod() (GH-120287)

In some cases, previously computed as (nan+nanj), we could recover
meaningful component values in the result, see e.g. the C11, Annex
G.5.1, routine _Cmultd():

>>> z = 1e300+1j
>>> z*(nan+infj)  # was (nan+nanj)
(-inf+infj)

That also fix some complex powers for small integer exponents, computed
with optimized algorithm (by squaring):

>>> z**5  # was (nan+nanj)
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    z**5
    ~^^~
OverflowError: complex exponentiation