ISTM that if frame->f_exc_type == NULL then exc_value
and exc_traceback will also be NULL.  I didn't see that
this is documented, perhaps I missed it or there is
some case when this can occur.  If it can occur, we
shoul develop a test for it.

Assuming this condition is invariant, some
simplifications can be made in reset_exc_info which is
called once per eval_frame (on function exit).

Also, I think there is currently an extra Py_INCREF on
Py_None.  This occurs when tstate->exc_type == NULL.

This patch seems to have little to no effect on
performance.  I did measure a 0.3% speed improvement.