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.
diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index 50701db..722e91e 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -2313,12 +2313,13 @@
     return data;
 }
 
-static void
+static inline void
 _PyMem_DebugCheckGIL(void)
 {
-    if (!PyGILState_Check())
+    if (!PyGILState_Check()) {
         Py_FatalError("Python memory allocator called "
                       "without holding the GIL");
+    }
 }
 
 static void *