ng_ipv6: set default values for src and dst in ng_ipv6_hdr_build()

Later in the handling of IPv6 headers, the source address is only
automatically chosen and filled into the header if the unspecified
address is found in the src field, but it's never set to that value.

The default destination address is set to '::1'.

sys/net/network_layer/ng_ipv6/hdr/ng_ipv6_hdr.c

@@ -63,6 +63,10 @@ ng_pktsnip_t *ng_ipv6_hdr_build(ng_pktsnip_t *payload,
 #endif
         memcpy(&hdr->src, src, src_len);
     }
+    else {
+        DEBUG("ipv6_hdr: set packet source to ::\n");
+        ng_ipv6_addr_set_unspecified(&hdr->src);
+    }
 
     memset(&hdr->dst + dst_len, 0, sizeof(ng_ipv6_addr_t) - dst_len);
 
@@ -74,6 +78,10 @@ ng_pktsnip_t *ng_ipv6_hdr_build(ng_pktsnip_t *payload,
 #endif
         memcpy(&hdr->dst, dst, dst_len);
     }
+    else {
+        DEBUG("ipv6_hdr: set packet destination to ::1\n");
+        ng_ipv6_addr_set_loopback(&hdr->dst);
+    }
 
     hdr->v_tc_fl = byteorder_htonl(0x60000000); /* set version, tc and fl in one go*/
     hdr->nh = NG_PROTNUM_RESERVED;