If the leakage rate is constant and your calculations say that it's impossible for the software to crash before the missile (heh), everything is fine. A missile

If the leakage rate is constant and your calculations say that it's impossible for the software to crash before the missile (heh), everything is fine. A missile has a maximum range because fuel is finite, so you can be sure it doesn't stay in the air longer than $t_max. Software doesn't have to be perfect and, for a matter of fact, it never is. It just has to perform it's purpose within the constraints it was designed for.