bgpd: update AS value of a hidden bgp instance

bgpd: update AS value of a hidden bgp instance

'import vrf VRF' could define a hidden bgp instance with
the default AS_UNSPECIFIED (i.e. = 1) value.
When a
	router bgp AS vrf VRF
gets configured later on, replace this AS_UNSPECIFIED setting
with a requested value.

Fixes: 9680831518 ("bgpd: fix as_pretty mem leaks when un-hiding")
Signed-off-by: Alexander Skorichenko <askorichenko@netgate.com>
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>
(cherry picked from commit 1515a59202280933936b41c4cb2cb11c7889b279)
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>