Fix autodiff test: need at least 1 residual.

internal/ceres/autodiff_test.cc

diff --git a/internal/ceres/autodiff_test.cc b/internal/ceres/autodiff_test.cc
index 86861ee..a9fb833 100644
--- a/internal/ceres/autodiff_test.cc
+++ b/internal/ceres/autodiff_test.cc
@@ -361,7 +361,7 @@
   // residuals.
   VaryingResidualFunctor functor;
 
-  for (int num_residuals = 0; num_residuals < kMaxResiduals; ++num_residuals) {
+  for (int num_residuals = 1; num_residuals < kMaxResiduals; ++num_residuals) {
     // Tweak the number of residuals to produce.
     functor.num_residuals = num_residuals;