Fix issues found by coverity

Fix issues CID:322884 and CID:322885.

Author: Arto Kinnunen

source/6LoWPAN/Thread/thread_extension_bbr.c

@@ -1087,7 +1087,7 @@ static int thread_border_router_pbbr_nw_data_register(thread_pbbr_t *this)
         return -1;
     }
 
-    if (!addr_is_ipv6_unspecified(this->domain_prefix)) {
+    if (memcmp(this->domain_prefix, ADDR_UNSPECIFIED, 8) != 0) {
         thread_border_router_info_t br_info = {0};
         br_info.P_on_mesh = true;
         br_info.stableData = true;
@@ -1443,7 +1443,7 @@ int thread_extension_bbr_prefix_set(int8_t interface_id, uint8_t *prefix)
         return 0;
     }
 
-    if (!addr_is_ipv6_unspecified(this->domain_prefix)) {
+    if (memcmp(this->domain_prefix, ADDR_UNSPECIFIED, 8) != 0) {
         thread_border_router_info_t br_info = {0};
         br_info.P_on_mesh = true;
         br_info.stableData = true;