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:
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REL8_0_STABLE

Modified Files:
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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)