rlc_um: add missing lock in get_buffer_state()

lib/src/upper/rlc_um.cc

@@ -274,6 +274,8 @@ bool rlc_um::rlc_um_tx_base::has_data()
 
 uint32_t rlc_um::rlc_um_tx_base::get_buffer_state()
 {
+  std::lock_guard<std::mutex> lock(mutex);
+
   // Bytes needed for tx SDUs
   uint32_t n_sdus  = tx_sdu_queue.size();
   uint32_t n_bytes = tx_sdu_queue.size_bytes();