igmp: leave groups in 224.0.0.0/24 to flooding

The switch keeps flooding 224.0.0.0/24 with IGMP on, but a report for a
group in that block still made an entry for it, and the entry limited
the group to the ports that reported it. A host announcing mDNS on
224.0.0.251 thereby cut mDNS off from every other port. Such reports
are now ignored, as RFC 4541 asks.

rtl837x_igmp.c

@@ -251,6 +251,8 @@ uint8_t igmp_packet_handler(void) __banked
 #ifdef DEBUG
 	print_string("IGMP join "); print_byte(igmp_join); write_char('\n');
 #endif
+	if (uip_buf[igmp_grp] == IGMP_LOCAL_NET_0 && !uip_buf[igmp_grp + 1] && !uip_buf[igmp_grp + 2])
+		return 1;
 
 #ifdef IPMC_USES_L3MC
 	memset((__xdata uint8_t *)&entry, 0, sizeof(struct ipmc_table_entry));