Remove usage of CHECK_NOTNULL

We want to switch to absl logging instead of glog.  gtest and ceres are
going there, and we already have absl as a dependency.  ABSL doesn't
have CHECK_NOTNULL, and we can move things over in an easier to review
fashion.

Change-Id: Ifd9a11ec34a2357cec43f88dba015db9c28ed2cf
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/frc971/vision/v4l2_reader.h b/frc971/vision/v4l2_reader.h
index a3d6252..4906d8e 100644
--- a/frc971/vision/v4l2_reader.h
+++ b/frc971/vision/v4l2_reader.h
@@ -107,8 +107,9 @@
     }
 
     absl::Span<const char> DataSpan(size_t image_size) {
-      return absl::Span<const char>(
-          reinterpret_cast<char *>(CHECK_NOTNULL(data_pointer)), image_size);
+      CHECK(data_pointer != nullptr);
+      return absl::Span<const char>(reinterpret_cast<char *>(data_pointer),
+                                    image_size);
     }
 
     aos::Sender<CameraImage> sender;