Fix and test (manually w/xx module) passing NULLs to PyObject_Str() and

Fix and test (manually w/xx module) passing NULLs to PyObject_Str() and
PyObject_Unicode().  This problem was originally reported from Coverity
and addresses mail on python-dev "checkin r43015".

This inlines the conversion of the string to unicode and cleans
up/simplifies some code at the end of the PyObject_Unicode().

We really need a complete C API test module for all public APIs
and passing good and bad parameter values.

Will backport.