ip-neighbor: GARP sent to bogus ip address

ip-neighbor: GARP sent to bogus ip address

The function ip4_neighbor_advertise may be called with NULL addr. In
that case, it looks up addr from fib by calling fib_sas4_get which
returns true or false to indicate whether there is an ip address
associated with the interface or not. But the caller to fib_sas4_get
does not check the return code and blindly assumes there is always an
ip address associated with the interface. As a result, it ends up
sending GARP to the bogus ip address if there is no ip address
associated with the interface.

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I7aa0270766c3943ed8ca8f8a092cae34567fd30e