Lora: Fix cancel_sending

This commit fixes some bugs from cancel_sending() method:
-  System crashed if method was called before initialization.
   Now LORAWAN_STATUS_NOT_INITIALIZED will be returned.
-  Method returned LORAWAN_STATUS_BUSY error when no send request was pending.
   LORAWAN_STATUS_OK should be returned in this case.
-  LORAWAN_STATUS_BUSY is now returned if backoff timer is just about to be
   dispatched (time_left returns 0).

features/lorawan/LoRaWANStack.cpp

@@ -277,15 +277,17 @@ lorawan_status_t LoRaWANStack::enable_adaptive_datarate(bool adr_enabled)
 
 lorawan_status_t LoRaWANStack::stop_sending(void)
 {
+    if (_device_current_state == DEVICE_STATE_NOT_INITIALIZED) {
+        return LORAWAN_STATUS_NOT_INITIALIZED;
+    }
+
     if (_loramac.clear_tx_pipe() == LORAWAN_STATUS_OK) {
-        if (_device_current_state == DEVICE_STATE_SENDING) {
         _ctrl_flags &= ~TX_DONE_FLAG;
         _ctrl_flags &= ~TX_ONGOING_FLAG;
         _loramac.set_tx_ongoing(false);
         _device_current_state = DEVICE_STATE_IDLE;
         return LORAWAN_STATUS_OK;
     }
-    }
 
     return LORAWAN_STATUS_BUSY;
 }

features/lorawan/lorastack/mac/LoRaMac.cpp

@@ -845,6 +845,7 @@ lorawan_status_t LoRaMac::handle_retransmission()
 void LoRaMac::on_backoff_timer_expiry(void)
 {
     Lock lock(*this);
+    _lora_time.stop(_params.timers.backoff_timer);
     lorawan_status_t status = schedule_tx();
     MBED_ASSERT(status == LORAWAN_STATUS_OK);
     (void) status;
@@ -1018,7 +1019,13 @@ int LoRaMac::get_backoff_timer_event_id(void)
 lorawan_status_t LoRaMac::clear_tx_pipe(void)
 {
     // check if the event is not already queued
-    if (_ev_queue->time_left(get_backoff_timer_event_id()) > 0) {
+    const int id = get_backoff_timer_event_id();
+    if (id == 0) {
+        // No queued send request
+        return LORAWAN_STATUS_OK;
+    }
+
+    if (_ev_queue->time_left(id) > 0) {
         _lora_time.stop(_params.timers.backoff_timer);
         _lora_time.stop(_params.timers.ack_timeout_timer);
         memset(_params.tx_buffer, 0, sizeof _params.tx_buffer);
@@ -1026,10 +1033,11 @@ lorawan_status_t LoRaMac::clear_tx_pipe(void)
         reset_ongoing_tx(true);
         tr_debug("Sending Cancelled");
         return LORAWAN_STATUS_OK;
-    }
-
+    } else {
+        // Event is already being dispatched so it cannot be cancelled
         return LORAWAN_STATUS_BUSY;
     }
+}
 
 lorawan_status_t LoRaMac::schedule_tx()
 {