Fixed njs_property_query() while quering symbol keys.

Fixed njs_property_query() while quering symbol keys.

Since 37f1f20de909, njs_object_hash_test() requires lhq->key.start
to be NULL to avoid hash collisions.

The field is zeroed by njs_property_query_init(), but the field still
might be non-zero when njs_property_query() is called inside a loop.

The fix is to ensure lhq->key.start == NULL for symbol keys.

This closes #268 issue on Github.