gnrc_ipv6: allow for empty IPv6 header

sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c

@@ -392,12 +392,17 @@ static int _fill_ipv6_hdr(gnrc_netif_t *netif, gnrc_pktsnip_t *ipv6)
 
     /* check if e.g. extension header was not already marked */
     if (hdr->nh == PROTNUM_RESERVED) {
-        hdr->nh = gnrc_nettype_to_protnum(ipv6->next->type);
-
-        /* if still reserved: mark no next header */
-        if (hdr->nh == PROTNUM_RESERVED) {
+        if (ipv6->next == NULL) {
             hdr->nh = PROTNUM_IPV6_NONXT;
         }
+        else {
+            hdr->nh = gnrc_nettype_to_protnum(ipv6->next->type);
+
+            /* if still reserved: mark no next header */
+            if (hdr->nh == PROTNUM_RESERVED) {
+                hdr->nh = PROTNUM_IPV6_NONXT;
+            }
+        }
     }
 
     DEBUG("ipv6: set next header to %u\n", hdr->nh);