Don't use mj_stackAlloc for int allocations.

PiperOrigin-RevId: 559846094
Change-Id: I43eeec0b1970b2c6047be4e7c3548a4ce9aa6f5f

src/engine/engine_core_constraint.c

@@ -1615,7 +1615,7 @@ static inline int mj_nc(const mjModel* m, mjData* d, int* nnz) {
   }
 
   mjMARKSTACK;
-  int *chain = (int*)mj_stackAlloc(d, m->nv);
+  int *chain = mj_stackAllocInt(d, m->nv);
 
   for (int i=0; i < ncon; i++) {
     if (d->contact[i].exclude) {

src/engine/engine_forward.c

@@ -774,8 +774,9 @@ void mj_implicitSkip(const mjModel* m, mjData* d, int skipfactor) {
       // set qLU = qM - dt*qDeriv
       mju_addToScl(d->qLU, d->qDeriv, -m->opt.timestep, m->nD);
 
-      // factorize qLU, use qacc as scratch space
-      mju_factorLUSparse(d->qLU, nv, (int*)qacc, d->D_rownnz, d->D_rowadr, d->D_colind);
+      // factorize qLU
+      int* scratch = mj_stackAllocInt(d, nv);
+      mju_factorLUSparse(d->qLU, nv, scratch, d->D_rownnz, d->D_rowadr, d->D_colind);
     }
 
     // solve for qacc: (qM - dt*qDeriv) * qacc = qfrc