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/aos/flatbuffers.h b/aos/flatbuffers.h
index e5de120..f1ed626 100644
--- a/aos/flatbuffers.h
+++ b/aos/flatbuffers.h
@@ -87,10 +87,12 @@
   }
 
   uint8_t *data() final {
-    return reinterpret_cast<uint8_t *>(CHECK_NOTNULL(data_));
+    CHECK(data_ != nullptr);
+    return reinterpret_cast<uint8_t *>(data_);
   }
   const uint8_t *data() const final {
-    return reinterpret_cast<const uint8_t *>(CHECK_NOTNULL(data_));
+    CHECK(data_ != nullptr);
+    return reinterpret_cast<const uint8_t *>(data_);
   }
   size_t size() const final { return size_; }