Patch some corner-case bugs in pl/python.

Dave Malcolm of Red Hat is working on a static code analysis tool for
Python-related C code.  It reported a number of problems in plpython,
most of which were failures to check for NULL results from object-creation
functions, so would only be an issue in very-low-memory situations.

Patch in HEAD and 9.1.  We could go further back but it's not clear that
these issues are important enough to justify the work.

Jan Urbański