Fix iterators for containers of NULL pointers (or Python None) when using -builtin.

Previously iteration would stop at the first element that had a NULL pointer value.

Examples/test-suite/python/li_std_vector_runme.py

@@ -2,3 +2,9 @@ from li_std_vector import *
 
 if typedef_test(101) != 101:
     raise RuntimeError
+
+try:
+  sv = StructVector([None, None])
+  raise RuntimeError("Using None should result in a TypeError")
+except TypeError:
+  pass