arp: checks for null in add neighbor

arp: checks for null in add neighbor

Type: fix

Details:

I have the situation that ip_neighbor_learn  is called for an already deleted interface.
The reproduction sequence is following
1.  arp_input -> arp_reply  on some worker
2.  call ip_neighbor_learn_dp , so request to perform  ip_neighbor_learn on the vpp_main thread
3.  the vpp_main thread is very busy - at the same moment we remove most of l2 interfaces and vrfs under barrier sync, including the TX interface of arp_reply
4.  call ip_neighbor_learn in the  main thread , when the appropriate interface is already deleted

Change-Id: I69b167ba919d57f19d6b941260243bca889c31c1
Signed-off-by: Anna Neiman <[email protected]>