rlc_am_lte: add some more sanity checks around retx of PDUs

this patch doesn't change any functionality but adds a bit
of further error checking around the handling of status PDUs
and the retx of PDUs

lib/src/upper/rlc_am_lte.cc

@@ -535,7 +535,16 @@ void rlc_am_lte::rlc_am_lte_tx::timer_expired(uint32_t timeout_id)
 
 void rlc_am_lte::rlc_am_lte_tx::retransmit_pdu()
 {
-  if (not tx_window.empty()) {
+  if (tx_window.empty()) {
+    logger.warning("%s No PDU to retransmit.", RB_NAME);
+    return;
+  }
+
+  if (not tx_window.has_sn(vt_a)) {
+    logger.warning("%s Can't retransmit unexisting SN=%d.", RB_NAME, vt_a);
+    return;
+  }
+
   // select first PDU in tx window for retransmission
   rlc_amd_tx_pdu_t& pdu = tx_window[vt_a];
   logger.info("%s Schedule SN=%d for reTx.", RB_NAME, pdu.rlc_sn);
@@ -545,7 +554,6 @@ void rlc_am_lte::rlc_am_lte_tx::retransmit_pdu()
   retx.so_end          = pdu.buf->N_bytes;
   retx.sn              = pdu.rlc_sn;
 }
-}
 
 /****************************************************************************
  * Helper functions
@@ -1186,6 +1194,11 @@ void rlc_am_lte::rlc_am_lte_tx::handle_control_pdu(uint8_t* payload, uint32_t no
     i = (i + 1) % MOD;
   }
 
+  // Make sure vt_a points to valid SN
+  if (not tx_window.empty() && not tx_window.has_sn(vt_a)) {
+    logger.error("%s vt_a=%d points to invalid position in Tx window", RB_NAME, vt_a);
+  }
+
   if (not notify_info_vec.empty()) {
     // Remove all SDUs that were fully acked
     for (uint32_t acked_pdcp_sn : notify_info_vec) {