sparse.cc: fix build_nd_action_vector() result storage

The generated action vector was stored in the 'actions' member of the
NDSparseMatrix that was the last one that was built (remembered in the
global ndsm pointer variable).

Now, we store it in the 'actions' member of the mdp_ndsm that is
passed in, as intended.

This did not result in problems before because the call to
build_nd_action_vector() was always immediately after the
corresponding NDSparseMatrix was built.

prism/src/sparse/sparse.cc

@@ -837,11 +837,11 @@ NDSparseMatrix *build_sub_nd_sparse_matrix(DdManager *ddman, DdNode *mdp, DdNode
 
 //------------------------------------------------------------------------------
 
-// build nondeterministic (mdp) action vector to accompany a sparse matrix
+// Build nondeterministic (mdp) action vector to accompany a sparse matrix
 // (i.e. a vector containing for every state and nondet choice, an index
-//  into the list of all action labels). store it in the 'actions' field.
+// into the list of all action labels).
+// Store the resulting vector in the 'actions' member of the passed in NDSparseMatrix.
 // throws std::bad_alloc on out-of-memory
-
 void build_nd_action_vector(DdManager *ddman, DdNode *mdp, DdNode *trans_actions, NDSparseMatrix *mdp_ndsm, DdNode **rvars, DdNode **cvars, int num_vars, DdNode **ndvars, int num_ndvars, ODDNode *odd)
 {
 	int i, n, nm, nc;
@@ -924,7 +924,7 @@ void build_nd_action_vector(DdManager *ddman, DdNode *mdp, DdNode *trans_actions
 	delete[] submatrices;
 	delete[] matrices_bdds;
 
-	ndsm->actions = actions;
+	mdp_ndsm->actions = actions;
 }
 
 //------------------------------------------------------------------------------