core: return errors to outstanding requests when serial ports are closed

Prevents crashes when the callback info completes when the modem is
removed, plus it's the right thing to do anyway...

src/mm-serial-port.c

@@ -825,6 +825,25 @@ mm_serial_port_close (MMSerialPort *self)
     for (i = 0; i < g_queue_get_length (priv->queue); i++) {
         MMQueueData *item = g_queue_peek_nth (priv->queue, i);
 
+        if (item->callback) {
+            GError *error;
+            GByteArray *response;
+
+            g_warn_if_fail (MM_SERIAL_PORT_GET_CLASS (self)->handle_response != NULL);
+            error = g_error_new_literal (MM_SERIAL_ERROR,
+                                         MM_SERIAL_ERROR_SEND_FAILED,
+                                         "Serial port is now closed");
+            response = g_byte_array_sized_new (1);
+            g_byte_array_append (response, (const guint8 *) "\0", 1);
+            MM_SERIAL_PORT_GET_CLASS (self)->handle_response (self,
+                                                              response,
+                                                              error,
+                                                              item->callback,
+                                                              item->user_data);
+            g_error_free (error);
+            g_byte_array_free (response, TRUE);
+        }
+
         g_byte_array_free (item->command, TRUE);
         g_slice_free (MMQueueData, item);
     }