Add an errant CHECK_NOTNULL

Change-Id: I07851bd103cff8c79dafd4801295d02806b7bd0b

internal/ceres/gradient_checking_cost_function.cc

diff --git a/internal/ceres/gradient_checking_cost_function.cc b/internal/ceres/gradient_checking_cost_function.cc
index a2bfa1c..9542af7 100644
--- a/internal/ceres/gradient_checking_cost_function.cc
+++ b/internal/ceres/gradient_checking_cost_function.cc
@@ -74,7 +74,7 @@
         relative_precision_(relative_precision),
         extra_info_(extra_info),
         callback_(callback) {
-    CHECK_NOTNULL(callback_);
+    CHECK(callback_ != nullptr);
     const vector<int32_t>& parameter_block_sizes =
         function->parameter_block_sizes();
     *mutable_parameter_block_sizes() = parameter_block_sizes;