shared/gatt-client: Fix possible crash
shared/gatt-client: Fix possible crash

Similar to issue found with unit test it is necessary to protect
bt_gatt_client instance before calling the callback otherwise it may
destroy it causing a crash.

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