AVRCP: Fix not checking for media_player_controller_create
AVRCP: Fix not checking for media_player_controller_create

Now that the MediaPlayer1 interface is experimental the interface
registration may fail which return NULL, in that case there is no
point on register to any notifications.

mapfau/bluez,ComputeCycles/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,pkarasev3/bluez,pkarasev3/bluez,silent-snowman/bluez,mapfau/bluez,ComputeCycles/bluez,pkarasev3/bluez,mapfau/bluez,mapfau/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez