Slight adaptation of Michael Hudson's patch to test PyDict_Next()

Slight adaptation of Michael Hudson's patch to test PyDict_Next()
(with modification of existing dict elements!).

This is part of SF patch #409864: lazy fix for Pings bizarre scoping
crash.

The adaptation I made to Michael's patch was to change the error
handling to avoid masking other errors (moving the specific error
message to inside test_dict_inner()), and to insert a test for
dict==NULL at the start.