rlc_am_lte: when max retx is reached, remove SN from tx window

this fixes an issue when the RLC bearer isn't reset from RRC.
In this case, the RLC would retransmit the same PDU over and over
again despite the max retx counter being reached.

lib/src/upper/rlc_am_lte.cc

@@ -625,6 +625,12 @@ int rlc_am_lte::rlc_am_lte_tx::build_retx_pdu(uint8_t* payload, uint32_t nof_byt
     logger.warning("%s Signaling max number of reTx=%d for for SN=%d", RB_NAME, tx_window[retx.sn].retx_count, retx.sn);
     parent->rrc->max_retx_attempted();
     parent->pdcp->notify_failure(parent->lcid, tx_window[retx.sn].pdcp_sns);
+
+    // remove SN from Tx window, advance window
+    tx_window.remove_pdu(retx.sn);
+    vt_a  = (vt_a + 1) % MOD;
+    vt_ms = (vt_ms + 1) % MOD;
+    return 0;
   }
 
   logger.info(payload,