pdcp,lte: fix debug logging in PDCP, to avoid segmentation fault and crash

lib/src/pdcp/pdcp_entity_lte.cc

@@ -472,10 +472,14 @@ void pdcp_entity_lte::update_rx_counts_queue(uint32_t rx_count)
       rx_counts_info.pop_back();
       fmc++;
     }
+    if (not rx_counts_info.empty()) {
       logger.debug("Queue too large. Updating. New FMC=%d, new back=%d, new queue_size=%zu",
                    fmc,
                    rx_counts_info.back(),
                    rx_counts_info.size());
+    } else {
+      logger.debug("Queue too large. Updating. New FMC=%d, new queue_size=%zu", fmc, rx_counts_info.size());
+    }
   }
 
   if (rx_counts_info.empty()) {