Issue#2238: some syntax errors from *args or **kwargs expressions

would give bogus error messages, because of untested exceptions::

    >>> f(**g(1=2))
    XXX undetected error
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: 'int' object is not iterable

instead of the expected SyntaxError: keyword can't be an expression

Will backport.

Python/ast.c

@@ -1934,10 +1934,14 @@ ast_for_call(struct compiling *c, const node *n, expr_ty func)
         }
         else if (TYPE(ch) == STAR) {
             vararg = ast_for_expr(c, CHILD(n, i+1));
+            if (!vararg)
+                return NULL;
             i++;
         }
         else if (TYPE(ch) == DOUBLESTAR) {
             kwarg = ast_for_expr(c, CHILD(n, i+1));
+            if (!kwarg)
+                return NULL;
             i++;
         }
     }