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/constants/constants_sender_lib.h b/frc971/constants/constants_sender_lib.h
index 9bd2c00..203435a 100644
--- a/frc971/constants/constants_sender_lib.h
+++ b/frc971/constants/constants_sender_lib.h
@@ -82,7 +82,10 @@
     });
   }
 
-  const ConstantsData &constants() const { return *fetcher_.get(); }
+  const ConstantsData &constants() const {
+    CHECK(fetcher_.get() != nullptr);
+    return *fetcher_.get();
+  }
 
  private:
   aos::Fetcher<ConstantsData> fetcher_;