pgsql: Backpatch to 8.0.X: In several places PL/Python was calling

Log Message:
-----------
Backpatch to 8.0.X:

In several places PL/Python was calling PyObject_Str() and then
PyString_AsString() without checking if the former had returned
NULL to indicate an error.  PyString_AsString() doesn't expect a
NULL argument, so passing one causes a segmentation fault.  This
patch adds checks for NULL and raises errors via PLy_elog(), which
prints details of the underlying Python exception.  The patch also
adds regression tests for these checks.  All tests pass on my
Solaris 9 box running HEAD and Python 2.4.1.

Tags:
----
REL8_0_STABLE

Modified Files:
--------------
    pgsql/src/pl/plpython:
        plpython.c (r1.58.4.1 -> r1.58.4.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c.diff?r1=1.58.4.1&r2=1.58.4.2)