libretro: Allow frontends to call disk_set_eject_state with no disk attached

[obbardc]

Some frontends (e.g. Retro-GTK) try to unmount a disk before setting the path for a new disk; let's be extra careful in the emulator and check if a disk really exists before attempting to unmount it.

Signed-off-by: Christopher Obbard chris.obbard@collabora.com

[realnc]

Thanks!