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.