af-packet: Error on invalid peer device setting

If the copy-iface was set to the same as the af-packet interface, the
peer won't be set. This leads to segfault. Catch it and return properly.

Bug 5870

src/source-af-packet.c

@@ -661,6 +661,10 @@ static void AFPWritePacket(Packet *p, int version)
     }
 
     /* Index of the network device */
+    if (p->afp_v.peer == NULL) {
+        SCLogError("Peer network device invalid");
+        return;
+    }
     socket_address.sll_ifindex = SC_ATOMIC_GET(p->afp_v.peer->if_idx);
     /* Address length*/
     socket_address.sll_halen = ETH_ALEN;