rlc_am_lte: replace simple sanitfy check with error log with expect

this should help debugging the situation when this happens by
aborting when STOP_ON_WARNING is enabled

lib/src/upper/rlc_am_lte.cc

@@ -1254,8 +1254,8 @@ void rlc_am_lte::rlc_am_lte_tx::handle_control_pdu(uint8_t* payload, uint32_t no
   }
 
   // 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 tx_window.empty()) {
+    srsran_expect(tx_window.has_sn(vt_a), "%s vt_a=%d points to invalid position in Tx window", RB_NAME, vt_a);
   }
 
   debug_state();