pgsql: Catch null pointer returns from PyCObject_AsVoidPtr and

Log Message:
-----------
Catch null pointer returns from PyCObject_AsVoidPtr and PyCObject_FromVoidPtr

This is reproducibly possible in Python 2.7 if the user turned
PendingDeprecationWarning into an error, but it's theoretically also possible
in earlier versions in case of exceptional conditions.

backpatched to 8.0

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql/src/pl/plpython:
plpython.c (r1.66.2.9 -> r1.66.2.10)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c?r1=1.66.2.9&r2=1.66.2.10)