android/avdtp: Fix crash when DISCOVER failed
android/avdtp: Fix crash when DISCOVER failed

Discover callback is only set when avdtp_discover was successful so we
need to check if it's set before calling.

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