bpo-38733: PyErr_Occurred() caller must hold the GIL (GH-17080)

bpo-3605, bpo-38733: Optimize _PyErr_Occurred(): remove "tstate ==
NULL" test.

Py_FatalError() no longer calls PyErr_Occurred() if called without
holding the GIL. So PyErr_Occurred() no longer has to support
tstate==NULL case.

_Py_CheckFunctionResult(): use directly _PyErr_Occurred() to avoid
explicit "!= NULL" test.

Python/errors.c

@@ -218,6 +218,9 @@ PyErr_SetString(PyObject *exception, const char *string)
 PyObject* _Py_HOT_FUNCTION
 PyErr_Occurred(void)
 {
+    /* The caller must hold the GIL. */
+    assert(PyGILState_Check());
+
     PyThreadState *tstate = _PyThreadState_GET();
     return _PyErr_Occurred(tstate);
 }