Patch 1145039. set_exc_info(), reset_exc_info(): By exploiting the likely (who knows?) invariant that when an exception's `type` is NULL, its `value` and `traceback` are also NULL, save some cycles in heavily-executed code. This is a "a kronar saved is a kronar earned" patch: the speedup isn't reliably measurable, but it obviously does reduce the operation count in the normal (no exception raised) path through PyEval_EvalFrameEx(). The tim-exc_sanity branch tries to push this harder, but is still blowing up (at least in part due to pre-existing subtle bugs that appear to have no other visible consequences!). Not a bugfix candidate.