Fix some places where we might fail to do Py_DECREF() on a Python
object (thereby leaking it for the rest of the session).  Almost
all of the risks were in error-recovery paths, which we don't really
expect to hit anyway.  Hence, while this is definitely a bug fix,
it doesn't quite seem worth back-patching.

Nikita Glukhov, Michael Paquier, Tom Lane

Discussion: https://postgr.es/m/28053a7d-10d8-fc23-b05c-b4749c873f63@postgrespro.ru