bpo-38631: Replace Py_FatalError() with assert() in ceval.c (GH-18279)

Replace a few Py_FatalError() calls if tstate is NULL with
assert(tstate != NULL) in ceval.c.

PyEval_AcquireThread(), PyEval_ReleaseThread() and
PyEval_RestoreThread() must never be called with a NULL tstate.
diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index 8913dbf..7ea48ae 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -1110,7 +1110,7 @@
 .. c:function:: void PyEval_AcquireThread(PyThreadState *tstate)
 
    Acquire the global interpreter lock and set the current thread state to
-   *tstate*, which should not be ``NULL``.  The lock must have been created earlier.
+   *tstate*, which must not be ``NULL``.  The lock must have been created earlier.
    If this thread already has the lock, deadlock ensues.
 
    .. note::