rlc_am: fix bug where PDU was erased from tx_window too early

lib/src/upper/rlc_am.cc

@@ -1069,20 +1069,19 @@ void rlc_am::handle_control_pdu(uint8_t *payload, uint32_t nof_bytes)
       //ACKed SNs get marked and removed from tx_window if possible
       if(tx_window.count(i) > 0) {
         it = tx_window.find(i);
-        it->second.is_acked = true;
+        if (it != tx_window.end()) {
+          if(update_vt_a) {
+            tx_window.erase(it);
             if(it->second.buf) {
               pool->deallocate(it->second.buf);
               it->second.buf = 0;
-          log->info("SN=%d removed from tx_window\n", i);
             }
-        tx_window.erase(it);
-        if(update_vt_a)
-        {
             vt_a = (vt_a + 1)%MOD;
             vt_ms = (vt_ms + 1)%MOD;
           }
         }
       }
+    }
     i = (i+1)%MOD;
   }