pgsql: Defend against possible crash if a plpython function does not

Log Message:
-----------
Defend against possible crash if a plpython function does not specify names
for its arguments.  Also add a regression test, since someone apparently
changed every single plpython test case to use only named parameters; else
we'd have noticed this sooner.

Euler Taveira de Oliveira, per a report from Alvaro

Modified Files:
--------------
    pgsql/src/pl/plpython/expected:
        plpython_function.out (r1.11 -> r1.12)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_function.out?r1=1.11&r2=1.12)
        plpython_test.out (r1.7 -> r1.8)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_test.out?r1=1.7&r2=1.8)
    pgsql/src/pl/plpython:
        plpython.c (r1.119 -> r1.120)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c?r1=1.119&r2=1.120)
    pgsql/src/pl/plpython/sql:
        plpython_function.sql (r1.11 -> r1.12)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/sql/plpython_function.sql?r1=1.11&r2=1.12)
        plpython_test.sql (r1.4 -> r1.5)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/sql/plpython_test.sql?r1=1.4&r2=1.5)