Add internal utility for zeroing int vectors.

Remove unnecessary branches in related functions.

PiperOrigin-RevId: 562768873
Change-Id: I3964a7a953d95bcd4e29137a7b590be5ffa6e311

src/engine/engine_io.c

@@ -874,7 +874,7 @@ static void makeDSparse(const mjModel* m, mjData* d) {
   }
 
   // populate colind
-  memcpy(remaining, rownnz, nv * sizeof(int));
+  mju_copyInt(remaining, rownnz, nv);
   for (int i = nv - 1; i >= 0; i--) {
     // init at diagonal
     remaining[i]--;