gnrc_rpl: fix zeroing of RPL DIS fields

The code originally assumed that the location of DIS struct is directly
after the ICMPv6 struct. This is not necessarily true when both structs
are individually allocated by pktbuf. This commit fixes this issue by
directly accessing the location of the DIS struct.

sys/net/gnrc/routing/rpl/gnrc_rpl_control_messages.c

@@ -314,7 +314,6 @@ void gnrc_rpl_send_DIS(gnrc_rpl_instance_t *inst, ipv6_addr_t *destination,
                        gnrc_rpl_internal_opt_t **options, size_t num_opts)
 {
     gnrc_pktsnip_t *pkt = NULL, *tmp;
-    icmpv6_hdr_t *icmp;
     gnrc_rpl_dis_t *dis;
 
     /* No options provided to be attached to the DIS, so we PadN 2 bytes */
@@ -358,6 +357,9 @@ void gnrc_rpl_send_DIS(gnrc_rpl_instance_t *inst, ipv6_addr_t *destination,
         return;
     }
     pkt = tmp;
+    dis = (gnrc_rpl_dis_t *)pkt->data;
+    dis->flags = 0;
+    dis->reserved = 0;
 
     if ((tmp = gnrc_icmpv6_build(pkt, ICMPV6_RPL_CTRL, GNRC_RPL_ICMPV6_CODE_DIS,
                                  sizeof(icmpv6_hdr_t))) == NULL) {
@@ -366,12 +368,6 @@ void gnrc_rpl_send_DIS(gnrc_rpl_instance_t *inst, ipv6_addr_t *destination,
         return;
     }
     pkt = tmp;
-
-    icmp = (icmpv6_hdr_t *)pkt->data;
-    dis = (gnrc_rpl_dis_t *)(icmp + 1);
-    dis->flags = 0;
-    dis->reserved = 0;
-
 #ifdef MODULE_NETSTATS_RPL
     gnrc_rpl_netstats_tx_DIS(&gnrc_rpl_netstats, gnrc_pkt_len(pkt),
                              (destination && !ipv6_addr_is_multicast(destination)));