android: Protect against null callbacks

According to the Android camera HAL C interface documentation, the
camera service is supposed to set callbacks after initializing the HAL
and calling get_number_of_cameras(), before any other calls to the
module. We rely on this behaviour and use callbacks unconditionally,
which would lead to a crash if the camera service behaved incorrectly.

While the camera service isn't supposed to behave incorrectly,
gracefully handling the error when opening cameras isn't costly, and
provides better diagnostic than a crash.

While at it, removed an unneeded [[maybe_unused]] attribute.

Reported-by: Coverity CID=298638
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Umang Jain <email@uajain.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/android/camera_hal_manager.cpp

@@ -29,7 +29,7 @@ LOG_DECLARE_CATEGORY(HAL);
  */
 
 CameraHalManager::CameraHalManager()
-	: cameraManager_(nullptr), numInternalCameras_(0),
+	: cameraManager_(nullptr), callbacks_(nullptr), numInternalCameras_(0),
 	  nextExternalCameraId_(firstExternalCameraId_)
 {
 }
@@ -70,6 +70,11 @@ CameraDevice *CameraHalManager::open(unsigned int id,
 {
 	MutexLocker locker(mutex_);
 
+	if (!callbacks_) {
+		LOG(HAL, Error) << "Can't open camera before callbacks are set";
+		return nullptr;
+	}
+
 	CameraDevice *camera = cameraDeviceFromHalId(id);
 	if (!camera) {
 		LOG(HAL, Error) << "Invalid camera id '" << id << "'";