Control: fixed a bug in control.

modules/control/common/interpolation_2d.cc

@@ -52,7 +52,9 @@ double Interpolation2D::Interpolate(const KeyType &xy) const {
 
   auto itr_after = xyz_.lower_bound(xy.first);
   auto itr_before = itr_after;
-  --itr_before;
+  if (itr_before != xyz_.begin()) {
+    --itr_before;
+  }
 
   double x_before = itr_before->first;
   double z_before = InterpolateYz(itr_before->second, xy.second);