Fix pydoc null pointer dereference with missing arg type

Processing doxygen @param comments for a parameter whose name did not
appear in the function declaration would cause a segfault due to a
null pointer dereference.

Adding test cases for both variadic function (no specified arguments)
and @param comment that references an argument that is not named in
the function prototype.  Both of these cases previously segfaulted.

Examples/test-suite/python/doxygen_basic_translate_runme.py

@@ -70,6 +70,16 @@ Test for a parameter with difficult type
 :type a: :py:class:`Shape`
 :param a: Very strange param"""
 )
+comment_verifier.check(inspect.getdoc(doxygen_basic_translate.function8),
+    """\
+Test variadic function
+:param ...: extra args"""
+)
+comment_verifier.check(inspect.getdoc(doxygen_basic_translate.function9),
+    """\
+Test unnamed argument
+:param baz: Description of baz"""
+)
 
 comment_verifier.check(inspect.getdoc(doxygen_basic_translate.Atan2),
     """\