Don't send wheel event with no flag set. Fix disconnection on touchpad scroll. BUG: 485726
While investigating BUG 485726 it was noticed that scrolling using a touchpad causes two empty QWheelEvents, where both angleDelta().x() and angleDelta().y() are set to 0. This caused a call to freerdp_input_send_mouse_event() with flags==0 that is rejected by the remote server and forces a disconnect.