camerad: fix VisionBuf freeing (#34264)

Co-authored-by: Comma Device <device@comma.ai>

system/camerad/cameras/camera_common.cc

@@ -93,9 +93,11 @@ void CameraBuf::init(cl_device_id device_id, cl_context context, SpectraCamera *
 }
 
 CameraBuf::~CameraBuf() {
+  if (camera_bufs_raw != nullptr) {
     for (int i = 0; i < frame_buf_count; i++) {
       camera_bufs_raw[i].free();
     }
+  }
   if (imgproc) delete imgproc;
 }