py: Fix overriding of default arguments.

Addresses issue #327.
diff --git a/py/objfun.c b/py/objfun.c
index a94978e..80cee16 100644
--- a/py/objfun.c
+++ b/py/objfun.c
@@ -249,7 +249,7 @@
         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--;
             }
         }