audio/player: Fix crash when current item UID is set to 0

audio/player: Fix crash when current item UID is set to 0

Apparently some stacks set the current UID to 0 when paused/stopped
which causes the following:

bluetoothd[23185]: profiles/audio/player.c:media_player_set_playlist_item() 0
bluetoothd[23185]: profiles/audio/player.c:media_folder_create_item() (null) type audio uid 0
process 23185: arguments to dbus_message_iter_append_basic() were incorrect, assertion "_dbus_check_is_valid_path (*string_p)" failed in file dbus-message.c line 2531.
This is normally a bug in some application using the D-Bus library.
  D-Bus not built with -rdynamic so unable to print a backtrace

UID 0 is not a valid UID according to the spec so the code should not
attempt to create any object to represent it.