lib,rlc_am_{lte,nr}: fix/add tx_window overrun protection

This commit fixes as bug causing a crash of the eNB in case
of many pending RETX and the reception of a trimmed PDU.
The following sequence leads to a crash:
- RETX queue contains many PDUs for RETX
- Receive trimmed PDU containing a trimmed subset of NACKs
- RETX queue is cleared and re-populated with a trimmed subset
- After all RETX (/!\ trimmed subset) is done, continue TX new PDUs
- tx_window blows up
- tx_window overflows if another status PDU is not received in time
- Overflow overwrites oldest element in tx_window
- Handling of next status PDU fails due to missing elements in tx_window

Related PR #4029

lib/src/rlc/rlc_am_lte.cc

@@ -690,8 +690,8 @@ int rlc_am_lte_tx::build_data_pdu(uint8_t* payload, uint32_t nof_bytes)
   }
 
   // do not build any more PDU if window is already full
-  if (tx_sdu == NULL && tx_window.size() >= RLC_AM_WINDOW_SIZE) {
-    RlcInfo("Tx window full.");
+  if (tx_window.size() >= RLC_AM_WINDOW_SIZE) {
+    RlcInfo("Cannot build data PDU - Tx window full.");
     return 0;
   }
 

lib/src/rlc/rlc_am_nr.cc

@@ -175,6 +175,13 @@ uint32_t rlc_am_nr_tx::build_new_pdu(uint8_t* payload, uint32_t nof_bytes)
     RlcInfo("Not enough bytes for payload plus header. nof_bytes=%d", nof_bytes);
     return 0;
   }
+
+  // do not build any more PDU if window is already full
+  if (tx_window->size() >= RLC_AM_WINDOW_SIZE) {
+    RlcInfo("Cannot build data PDU - Tx window full.");
+    return 0;
+  }
+
   // Read new SDU from TX queue
   unique_byte_buffer_t tx_sdu;
   RlcDebug("Reading from RLC SDU queue. Queue size %d", tx_sdu_queue.size());