Add internal utility for zeroing int vectors.

Remove unnecessary branches in related functions.

PiperOrigin-RevId: 562768873
Change-Id: I3964a7a953d95bcd4e29137a7b590be5ffa6e311

src/engine/engine_util_misc.c

@@ -1209,9 +1209,14 @@ int mju_isZero(mjtNum* vec, int n) {
 
 // set integer vector to 0
 void mju_zeroInt(int* res, int n) {
-  if (n > 0) {
-    memset(res, 0, n*sizeof(int));
-  }
+  memset(res, 0, n*sizeof(int));
+}
+
+
+
+// copy int vector vec into res
+void mju_copyInt(int* res, const int* vec, int n) {
+  memcpy(res, vec, n*sizeof(int));
 }