ASPA: fix aspa_check_upstream and aspa_check_downstream parse crash

ASPA: fix aspa_check_upstream and aspa_check_downstream parse crash

Due to internal differences between BIRD 2 and 3, there is
FI_CURRENT_ROUTE in BIRD 3 and `val.rte == NULL` is not supported
as a shortcut for fetching the route object.

When merging, we forgot to fix this difference in the BGP config parser,
thus the constant folder failed to recognize that this is not a constant
expression, leading to accidentally dereference the NULL route pointer
in config parse time.

This fixes #247.