BGP: Do not get out of bgp_start() if peer's IP address isn't known

BGP: Do not get out of bgp_start() if peer's IP address isn't known

Ticket: CM-7140
Reviewed By: CCR-3412
Testing Done: bgpsmoke, Atul verified fix

BGP Unnumbered and Interface based peering can interact in some strange
ways. One of them is when there's an IPv4 address on a link on which
BGP Unnumbered session is beng attempted, but the IPv4 address is not
a /30 or /31. As per the bug report, we end up attempting to start the
BGP FSM on receiving a notification that an IPv4 address is present on
an interface. To avoid attempting to go past BGP's start state in the
absence of a valid peer address is the right thing to do. And this
simple patch does just that.

Signed-off-by: Dinesh G Dutt <ddutt@cumulusnetworks.com>
Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Vipin Kumar <vipin@cumulusnetworks.com>