Add internal utility for zeroing int vectors.

Remove unnecessary branches in related functions.

PiperOrigin-RevId: 562768873
Change-Id: I3964a7a953d95bcd4e29137a7b590be5ffa6e311

src/engine/engine_util_misc.h

@@ -122,6 +122,9 @@ MJAPI int mju_isZero(mjtNum* vec, int n);
 // set integer vector to 0
 MJAPI void mju_zeroInt(int* res, int n);
 
+// copy int vector vec into res
+MJAPI void mju_copyInt(int* res, const int* vec, int n);
+
 // standard normal random number generator (optional second number)
 MJAPI mjtNum mju_standardNormal(mjtNum* num2);