Fix a pointer access bug in Ridders' algorithm.

A pointer to an Eigen matrix was being used as an array.

Change-Id: Ifaea14fa3416eda5953de49afb78dc5a6ea816eb

include/ceres/internal/numeric_diff.h

diff --git a/include/ceres/internal/numeric_diff.h b/include/ceres/internal/numeric_diff.h
index 7034a2a..11e8275 100644
--- a/include/ceres/internal/numeric_diff.h
+++ b/include/ceres/internal/numeric_diff.h
@@ -346,7 +346,7 @@
 
       // Store initial results.
       if (i == 0) {
-        residuals = current_candidates[0];
+        residuals = current_candidates->col(0);
       }
 
       // Shrink differentiation step size.