Matrix::conservativeResize, resize only when necessary.

Eigen/src/Core/Matrix.h

diff --git a/Eigen/src/Core/Matrix.h b/Eigen/src/Core/Matrix.h
index 2e28262..027e6bb 100644
--- a/Eigen/src/Core/Matrix.h
+++ b/Eigen/src/Core/Matrix.h
@@ -707,6 +707,8 @@
 {
   static void run(MatrixBase<Derived>& _this, const MatrixBase<OtherDerived>& other)
   {
+    if (_this.rows() == other.rows() && _this.cols() == other.cols()) return;
+
     // Note: Here is space for improvement. Basically, for conservativeResize(int,int),
     // neither RowsAtCompileTime or ColsAtCompileTime must be Dynamic. If only one of the
     // dimensions is dynamic, one could use either conservativeResize(int rows, NoChange_t) or
@@ -728,6 +730,8 @@
 {
   static void run(MatrixBase<Derived>& _this, const MatrixBase<OtherDerived>& other)
   {
+    if (_this.rows() == other.rows() && _this.cols() == other.cols()) return;
+
     // segment(...) will check whether Derived/OtherDerived are vectors!
     typename MatrixBase<Derived>::PlainMatrixType tmp(other);
     const int common_size = std::min<int>(_this.size(),tmp.size());