py: Fix overriding of default arguments.

Addresses issue #327.

py/objfun.c

@@ -249,7 +249,7 @@ continue2:;
         mp_obj_t *d = &flat_args[self->n_args - 1];
         mp_obj_t *s = &self->extra_args[self->n_def_args - 1];
         for (int i = self->n_def_args; i > 0; i--) {
-            if (*d != MP_OBJ_NULL) {
+            if (*d == MP_OBJ_NULL) {
                 *d-- = *s--;
             }
         }

tests/basics/fun-defargs2.py

+# overriding default arguments
+
+def foo(a, b=3):
+    print(a, b)
+
+# override with positional
+foo(1, 333)
+
+# override with keyword
+foo(1, b=333)
+
+# override with keyword
+foo(a=2, b=333)