android/gatt: Fix possible NULL dereference
android/gatt: Fix possible NULL dereference

Fixes warnings:
...
Pointer 'req' returned from call to function 'queue_peek_head'
may be NULL and will be dereferenced
...

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