Check for unregistered video factory in lookup_dev(#4129)

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This prevents lookup_dev()in pjmedia/src/pjmedia/videodev.c form look for PJMEDIA_VID_DEFAULT_CAPTURE_DEVand PJMEDIA_VID_DEFAULT_RENDER_DEV in drivers unregistered with pjmedia_vid_unregister_factory(). This affects to such high-level functions as: pjmedia_vid_dev_get_local_index, pjmedia_vid_dev_get_info, pjmedia_vid_dev_default_param, pjmedia_vid_dev_stream_create, pjmedia_vid_dev_stream_set_cap