rlc_um: fix PDU packing when newly added SDU segment was too large

during the PDU packing it could happen that a new SDU segment
was added but the resulting larger header was so big that not even
a single byte of the new PDU could be added. because this
corner case wasn't handled correctly an invalid PDU was transmitted.

the solution is to revert the addition of the new SDU.

lib/src/upper/rlc_um_base.cc

@@ -289,7 +289,7 @@ int rlc_um_base::rlc_um_base_tx::build_data_pdu(uint8_t* payload, uint32_t nof_b
     std::lock_guard<std::mutex> lock(mutex);
     log->debug("MAC opportunity - %d bytes\n", nof_bytes);
 
-    if (!tx_sdu && tx_sdu_queue.size() == 0) {
+    if (tx_sdu == nullptr && tx_sdu_queue.is_empty()) {
       log->info("No data available to be sent\n");
       return 0;
     }

lib/src/upper/rlc_um_lte.cc

@@ -167,12 +167,18 @@ int rlc_um_lte::rlc_um_lte_tx::build_data_pdu(unique_byte_buffer_t pdu, uint8_t*
   // Pull SDUs from queue
   while (pdu_space > head_len + 1 && tx_sdu_queue.size() > 0) {
     log->debug("pdu_space=%d, head_len=%d\n", pdu_space, head_len);
-    if (last_li > 0)
+    if (last_li > 0) {
       header.li[header.N_li++] = last_li;
+    }
     head_len       = rlc_um_packed_length(&header);
-    tx_sdu         = tx_sdu_queue.read();
     uint32_t space = pdu_space - head_len;
-    to_move        = space >= tx_sdu->N_bytes ? tx_sdu->N_bytes : space;
+    if (space == 0) {
+      // we cannot even fit a single byte of the newly added SDU, remove it again
+      header.N_li--;
+      break;
+    }
+    tx_sdu  = tx_sdu_queue.read();
+    to_move = (space >= tx_sdu->N_bytes) ? tx_sdu->N_bytes : space;
     log->debug("%s adding new SDU segment - %d bytes of %d remaining\n", rb_name.c_str(), to_move, tx_sdu->N_bytes);
     memcpy(pdu_ptr, tx_sdu->msg, to_move);
     last_li = to_move;
@@ -200,13 +206,12 @@ int rlc_um_lte::rlc_um_lte_tx::build_data_pdu(unique_byte_buffer_t pdu, uint8_t*
   // Add header and TX
   rlc_um_write_data_pdu_header(&header, pdu.get());
   memcpy(payload, pdu->msg, pdu->N_bytes);
-  uint32_t ret = pdu->N_bytes;
 
-  log->info_hex(payload, ret, "%s Tx PDU SN=%d (%d B)\n", rb_name.c_str(), header.sn, pdu->N_bytes);
+  log->info_hex(payload, pdu->N_bytes, "%s Tx PDU SN=%d (%d B)\n", rb_name.c_str(), header.sn, pdu->N_bytes);
 
   debug_state();
 
-  return ret;
+  return pdu->N_bytes;
 }
 
 void rlc_um_lte::rlc_um_lte_tx::debug_state()