pgsql: Avoid Python memory leaks in hstore_plpython and jsonb_plpython.

Avoid Python memory leaks in hstore_plpython and jsonb_plpython.

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9e360f0e8327bcf4b7c5ce957024fdff8f95f7de

Modified Files
--------------
contrib/hstore_plpython/hstore_plpython.c | 11 +--
contrib/jsonb_plpython/jsonb_plpython.c | 108 +++++++++++++++++++++---------
2 files changed, 83 insertions(+), 36 deletions(-)