gnrc_ipv6: don't dispatch encapsulated IPv6 headers in central function

Otherwise, an encapsulated IPv6 packet is handled twice. Once in the
central function, once in the specialized decapsulation.

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

@@ -213,9 +213,11 @@ static void _dispatch_next_header(gnrc_pktsnip_t *current, gnrc_pktsnip_t *pkt,
 {
 #ifdef MODULE_GNRC_IPV6_EXT
     const bool should_dispatch_current_type = ((current->type != GNRC_NETTYPE_IPV6_EXT) ||
-                                               (current->next->type == GNRC_NETTYPE_IPV6));
+                                               (current->next->type == GNRC_NETTYPE_IPV6)) &&
+                                              (current->type != GNRC_NETTYPE_IPV6);
 #else
-    const bool should_dispatch_current_type = (current->next->type == GNRC_NETTYPE_IPV6);
+    const bool should_dispatch_current_type = (current->next->type == GNRC_NETTYPE_IPV6) &&
+                                              (current->type != GNRC_NETTYPE_IPV6);
 #endif
 
     DEBUG("ipv6: forward nh = %u to other threads\n", nh);