audio/avrcp: Fix crash on RegisterNotification
audio/avrcp: Fix crash on RegisterNotification

When registering AVRCP_EVENT_ADDRESSED_PLAYER_CHANGED there may not be
any player available causing a crash.

ComputeCycles/bluez,ComputeCycles/bluez,pkarasev3/bluez,pkarasev3/bluez,pkarasev3/bluez,pkarasev3/bluez,ComputeCycles/bluez,ComputeCycles/bluez