Check object before usage (#2036)

Avoid potential crash by checking object availability before using it

Author: Arto Kinnunen

source/6LoWPAN/Thread/thread_extension_bbr.c

@@ -1529,9 +1529,13 @@ void thread_extension_bbr_status_override_set(uint8_t dua_status, uint8_t dua_co
 void thread_extension_bbr_mcast_fwd_check(int8_t interface_id, bool *multicast_fwd)
 {
     thread_pbbr_t *this = thread_bbr_find_by_interface(interface_id);
-    protocol_interface_info_entry_t *cur = protocol_stack_interface_info_get_by_id(this->interface_id);
 
-    if (!cur || !this || multicast_fwd) {
+    if (!this || multicast_fwd) {
+        return;
+    }
+
+    protocol_interface_info_entry_t *cur = protocol_stack_interface_info_get_by_id(this->interface_id);
+    if (!cur) {
         return;
     }