py: Add stack check to mp_iternext, since it can be called recursively.

Eg, builtin map can map over a map, etc, and call iternext deeply.

Addresses issue #1294.

py/runtime.c

@@ -1044,6 +1044,7 @@ mp_obj_t mp_iternext_allow_raise(mp_obj_t o_in) {
 // will always return MP_OBJ_STOP_ITERATION instead of raising StopIteration() (or any subclass thereof)
 // may raise other exceptions
 mp_obj_t mp_iternext(mp_obj_t o_in) {
+    MP_STACK_CHECK(); // enumerate, filter, map and zip can recursively call mp_iternext
     mp_obj_type_t *type = mp_obj_get_type(o_in);
     if (type->iternext != NULL) {
         return type->iternext(o_in);