[LinearSolver.Direct] BTDLinearSolver::partial_solve: fix members initialization/bad values

Sofa/Component/LinearSolver/Direct/src/sofa/component/linearsolver/direct/BTDLinearSolver.h

@@ -148,9 +148,9 @@ class BTDLinearSolver : public sofa::component::linearsolver::MatrixLinearSolver
 private:
 
 
-    Index _indMaxNonNullForce; // point with non null force which index is the greatest and for which globalAccumulate was not proceed
+    Index _indMaxNonNullForce{0}; // point with non null force which index is the greatest and for which globalAccumulate was not proceed
 
-    Index _indMaxFwdLHComputed;  // indice of node from which bwdLH is accurate
+    Index _indMaxFwdLHComputed{0};  // indice of node from which bwdLH is accurate
 
     /// private functions for partial solve
     /// step1=> accumulate RH locally for the InBloc (only if a new force is detected on RH)

Sofa/Component/LinearSolver/Direct/src/sofa/component/linearsolver/direct/BTDLinearSolver.inl

@@ -412,6 +412,11 @@ void BTDLinearSolver<Matrix,Vector>::init_partial_solve()
     // Block that is currently being proceed => start from the end (so that we use step2 bwdAccumulateLHGlobal and accumulate potential initial forces)
     current_bloc = nb-1;
 
+    // no force accumulated yet, and no LH computed during forward:
+    // both must be reset along with current_bloc, otherwise a value from the previous
+    // time step (or an out of range one if nb has decreased) is used in partial_solve
+    _indMaxNonNullForce = 0;
+    _indMaxFwdLHComputed = 0;
 
     // DF represents the variation of the right hand side of the equation (Force in mechanics)
     Vec_dRH.resize(nb);