Fix IntervalBuilder crash during a check deletion

As I said in #243, if we delete a check with a huge pings history, it's very long (Because of I/O mass operations, #245) and finally crash certainly because the `IntervalBuilder` try to calculate duration and/or downtime of a check that doesn't exist anymore.

I managed to find a workaround by invoking the calculation function only if check exists.

I don't know if is the best way but now, the very long delete processing works after all and the all of the uptime system seems to work properly.