Explicitly reset xipos for worldbody.

Before this change, xipos[0,1,2] was only set to 0 by mj_resetData zeroing the entire buffer.

When using mj_resetDataDebug, the value would be wrong.

PiperOrigin-RevId: 450939814
Change-Id: I887dd977161c7d132a80b766db7be23350ffb50b

src/engine/engine_core_smooth.c

@@ -40,6 +40,7 @@ void mj_kinematics(const mjModel* m, mjData* d) {
   // set world position and orientation
   mju_zero3(d->xpos);
   mju_unit4(d->xquat);
+  mju_zero3(d->xipos);
   mju_zero(d->xmat, 9);
   mju_zero(d->ximat, 9);
   d->xmat[0] = d->xmat[4] = d->xmat[8] = 1;