bgpd: Bug fix in "show bgp l2vpn evpn ... advertised-routes'

bgpd: Bug fix in "show bgp l2vpn evpn ... advertised-routes'

The bug:
As part of displaying advertised routes to a peer, in the outer loop, we
iterate through all prefixes in the evpn table. In the inner loop,
we iterate through adj_out of each prefix.

If a prefix which is present in the evpn table is not advertised to a peer,
its corresponding attr == NULL. Checking for this condition is the fix.

Signed-off-by: Lakshman Krishnamoorthy <lkrishnamoor@vmware.com>