bgpd: fix distance for aggregate route

bgpd: fix distance for aggregate route

bgp aggregate address installs route with self peer which
can have peer->su of unspecifed type.
bgp_distance_apply bailed out as it fails to parse
sockunion2hostprefix for af type unspec.

config:
 address-family ipv4 unicast
  aggregate-address 50.1.0.0/16 summary-only

Testing Done:

Before:
B>* 50.1.0.0/16 [20/0] unreachable (blackhole), weight 1, 00:00:02

After:
B>* 50.1.0.0/16 [200/0] unreachable (blackhole), weight 1, 00:01:28

Signed-off-by: Chirag Shah <chirag@nvidia.com>