py/map: Store key/value in earliest possible slot in hash table.

This change makes the code behave how it was supposed to work when first
written.  The avail_slot variable is set to the first free slot when
looking for a key (which would come from deleting an entry).  So it's
more efficient (for subsequent lookups) to insert a new key into such a
slot, rather than the very last slot that was searched.

py/map.c

@@ -214,12 +214,12 @@ mp_map_elem_t *mp_map_lookup(mp_map_t *map, mp_obj_t index, mp_map_lookup_kind_t
                 if (avail_slot == NULL) {
                     avail_slot = slot;
                 }
-                slot->key = index;
-                slot->value = MP_OBJ_NULL;
+                avail_slot->key = index;
+                avail_slot->value = MP_OBJ_NULL;
                 if (!MP_OBJ_IS_QSTR(index)) {
                     map->all_keys_are_qstrs = 0;
                 }
-                return slot;
+                return avail_slot;
             } else {
                 return NULL;
             }