Fixed incorrect mesh face circumradius calculation

- In 8e2f830, this was changed from `mju_norm3` to `mjuu_normvec`, but the
  latter will mutate its input to be normalized instead of just
returning the norm (the former was likely mistaken as `mju_normalize3`
which does mutate its input).

src/user/user_mesh.cc

@@ -1915,7 +1915,7 @@ void mjCMesh::MakeCenter(void) {
     // compute circumradius
     double norm_a_2 = mjuu_dot3(a, a);
     double norm_b_2 = mjuu_dot3(b, b);
-    double area = mjuu_normvec(nrm, 3);
+    double area = sqrt(mjuu_dot3(nrm, nrm));
 
     // compute circumcenter
     double res[3], vec[3] = {