[Symmetry]

If you were frequently catching out of bounds accesses your CPU's branch predictor would be making mistakes and you'd be frequently eating the branch mispredict penalty. Any extra instruction or check that alters the control flow is often much more expensive than just another addition. However, in this case if you're taking the branch something is seriously broken so that you should not be facing this. Pre-Haswell Intel CPUs only had one branch slot so you still might have a penalty but as you say this isn't the end of the world.