port-serial: fix command context completion when clearing command queue

This patch removes an extra invocation of g_simple_async_result_complete
on the result associated with each command context in the command queue
when mm_port_serial_close clears the queue. It also changes the code to
complete the results in idle, which avoids a nested invocation of
mm_port_serial_close. That could happen if the completion of the result
calls mm_port_serial_close again (e.g. via at_command_context_free,
at_sequence_context_free in mm-base-modem-at.c). The nested invocation
of mm_port_serial_close could create undesirable effects (e.g. the
assertion on open_count > 0 fails in case of a forced close).

src/mm-port-serial.c

@@ -1378,8 +1378,7 @@ mm_port_serial_close (MMPortSerial *self)
                                          MM_SERIAL_ERROR,
                                          MM_SERIAL_ERROR_SEND_FAILED,
                                          "Serial port is now closed");
-        g_simple_async_result_complete (ctx->result);
-        command_context_complete_and_free (ctx, FALSE);
+        command_context_complete_and_free (ctx, TRUE);
     }
     g_queue_clear (self->priv->queue);