Fixed #1573: Improper use of the keyword-only syntax makes the parser crash

>>> def f(*, **kw):
...   pass
...
python: Python/ast.c:652: handle_keywordonly_args: Assertion 'kwonlyargs
!= ((void *)0)' failed.

Python/ast.c

@@ -649,8 +649,8 @@ handle_keywordonly_args(struct compiling *c, const node *n, int start,
     arg_ty arg;
     int i = start;
     int j = 0; /* index for kwdefaults and kwonlyargs */
-    assert(kwonlyargs != NULL);
-    assert(kwdefaults != NULL);
+    assert((kwonlyargs != NULL && kwdefaults != NULL) ||
+           TYPE(CHILD(n, i)) == DOUBLESTAR);
     while (i < NCH(n)) {
         ch = CHILD(n, i);
         switch (TYPE(ch)) {