gnrc: add some assertions

sys/net/network_layer/ng_icmpv6/ng_icmpv6.c

@@ -62,10 +62,14 @@ void ng_icmpv6_demux(kernel_pid_t iface, ng_pktsnip_t *pkt)
 
     LL_SEARCH_SCALAR(pkt, icmpv6, type, NG_NETTYPE_ICMPV6);
 
+    assert(icmpv6 != NULL);
+
     /* there can be extension headers between IPv6 and ICMPv6 header so we have
      * to search it */
     LL_SEARCH_SCALAR(icmpv6, ipv6, type, NG_NETTYPE_IPV6);
 
+    assert(ipv6 != NULL);
+
     hdr = (ng_icmpv6_hdr_t *)icmpv6->data;
 
     if (_calc_csum(icmpv6, ipv6, pkt)) {

sys/net/network_layer/ng_ipv6/ng_ipv6.c

@@ -593,6 +593,8 @@ static void _receive(ng_pktsnip_t *pkt)
     ng_pktsnip_t *ipv6, *netif;
     ng_ipv6_hdr_t *hdr;
 
+    assert(pkt != NULL);
+
     LL_SEARCH_SCALAR(pkt, netif, type, NG_NETTYPE_NETIF);
 
     if (netif != NULL) {

sys/net/transport_layer/ng_udp/ng_udp.c

@@ -120,6 +120,8 @@ static void _receive(ng_pktsnip_t *pkt)
 
     LL_SEARCH_SCALAR(pkt, ipv6, type, NG_NETTYPE_IPV6);
 
+    assert(ipv6 != NULL);
+
     /* validate checksum */
     if (_calc_csum(udp, ipv6, pkt)) {
         DEBUG("udp: received packet with invalid checksum, dropping it\n");
@@ -144,6 +146,9 @@ static void _send(ng_pktsnip_t *pkt)
 
     /* get udp snip and hdr */
     LL_SEARCH_SCALAR(pkt, udp_snip, type, NG_NETTYPE_UDP);
+
+    assert(udp_snip != NULL);
+
     udp_snip = ng_pktbuf_start_write(udp_snip);
     if (udp_snip == NULL) {
         DEBUG("udp: cannot send packet: unable to allocate packet\n");