Properly initialize b vector in SplineFitting

InterpolateWithDerivative does not initialize the be vector correctly. This issue is discussed In stackoverflow question 48382939.

unsupported/Eigen/src/Splines/SplineFitting.h

diff --git a/unsupported/Eigen/src/Splines/SplineFitting.h b/unsupported/Eigen/src/Splines/SplineFitting.h
index 850f78a..9f6e8af 100644
--- a/unsupported/Eigen/src/Splines/SplineFitting.h
+++ b/unsupported/Eigen/src/Splines/SplineFitting.h
@@ -385,7 +385,7 @@
     {
       const DenseIndex span = SplineType::Span(parameters[i], degree, knots);
 
-      if (derivativeIndices[derivativeIndex] == i)
+      if (derivativeIndex < derivativeIndices.size() && derivativeIndices[derivativeIndex] == i)
       {
         A.block(row, span - degree, 2, degree + 1)
           = SplineType::BasisFunctionDerivatives(parameters[i], 1, degree, knots);
@@ -395,8 +395,9 @@
       }
       else
       {
-        A.row(row++).segment(span - degree, degree + 1)
+        A.row(row).segment(span - degree, degree + 1)
           = SplineType::BasisFunctions(parameters[i], degree, knots);
+        b.col(row++) = points.col(i);
       }
     }
     b.col(0) = points.col(0);