Increment ip.drop when dropping due to NULL netif

This commit increments the ip.drop statistic when an IP packet is
dropped due to no matching netif found and forwarding is disabled

This adds parity to the other places where mib2.ipinaddrerrors and
mib2.ipindiscards are incremented which also increment ip.drop

src/core/ipv4/ip4.c

@@ -598,6 +598,7 @@ ip4_input(struct pbuf *p, struct netif *inp)
     } else
 #endif /* IP_FORWARD */
     {
+      IP_STATS_INC(ip.drop);
       MIB2_STATS_INC(mib2.ipinaddrerrors);
       MIB2_STATS_INC(mib2.ipindiscards);
     }