vlib: fix crash on packet on deleted interface

vlib: fix crash on packet on deleted interface

If ip4_neighbor_probe (or any other) is sending packet to a deleted interface,
ASSERT trips and dataplane crashes. Example:

create loopback interface instance 0
set interface ip address loop0 10.0.0.1/32
set interface state GigabitEthernet3/0/1 up
set interface state loop0 up
set interface state loop0 down
set interface ip address del loop0 10.0.0.1/32
delete loopback interface intfc loop0
set interface state GigabitEthernet3/0/1 down
set interface state GigabitEthernet3/0/1 up
comment { the following crashes VPP }
set interface state GigabitEthernet3/0/1 down

This sequence reliably crashes VPP:

(gdb)p n->name
$4 = (u8 *) 0x7fff82b47578 "interface-3-output-deleted”

If the interface doesn't exist, return ~0 and be tolerant of this in the
two call sites of counter_index()

Type: fix
Signed-off-by: Pim van Pelt <pim@ipng.nl>
Change-Id: I90ec58fc0d14b20c9822703fe914f2ce89acb18d