libvlc_media_player_set_media segfaults when pass a null pointer for the new media

In VLC 3.x, you could pass a null pointer to libvlc_media_player_set_media to unset the media.

When the commit c0c35a50025df54cee95ddb00b325438b1f47e5e was merged, this ability was removed, and checks for null pointer were removed causing a segfault as elements of libvlc_media were accessed.