nas: do not abort attach procedure if RRC is already connected

this is an attempt to fix issue #2237 that cause the UE to fail to
attach because the attach request wasn't resent after T3410+T3411
expired.

this was only because RRC wasn't idle at that stage and the attach
request generation was aborted.

The patch fixes this by passing the NAS PDU directly to RRC in
connected state.

The patch also converts the blocking PDU alloc to a non-blocking one.

srsue/src/stack/upper/nas.cc

@@ -290,11 +290,6 @@ bool nas::disable_data()
  */
 void nas::start_attach_request(srslte::establishment_cause_t cause_)
 {
-  if (rrc->is_connected()) {
-    logger.info("RRC is connected. Aborting attach reuqest %s.", to_string(cause_).c_str());
-    return;
-  }
-
   logger.info("Attach Request with cause %s.", to_string(cause_).c_str());
 
   if (state.get_state() != emm_state_t::state_t::deregistered) {
@@ -318,9 +313,16 @@ void nas::start_attach_request(srslte::establishment_cause_t cause_)
   }
 
   // Start attach request
-  unique_byte_buffer_t msg = srslte::allocate_unique_buffer(*pool, true);
+  unique_byte_buffer_t msg = srslte::allocate_unique_buffer(*pool);
+  if (msg == nullptr) {
+    logger.warning("Couldn't allocate buffer for Attach request.\n");
+    return;
+  }
+
   gen_attach_request(msg);
-  if (!rrc->is_connected()) {
+  if (rrc->is_connected()) {
+    rrc->write_sdu(std::move(msg));
+  } else {
     logger.debug("Initiating RRC Connection");
     if (not rrc->connection_request(cause_, std::move(msg))) {
       logger.error("Error starting RRC connection");