PyErr_NormalizeException(): in the type==NULL test, we should simply

PyErr_NormalizeException(): in the type==NULL test, we should simply
return.  Setting an exception can mess with the exception state, and
continuing is definitely wrong (since type is dereferenced later on).
Some code that calls this seems to be prepared for a NULL exception
type, so let's be safe rather than sorry and simply assume there's
nothing to normalize in this case.