Minefield: ignore the clock after we explode

If we attempt a multiclear by clicking on a number location that
has enough flags and that explodes, we will still get releases
for the other location around the clicked one, but we must make
sure they don't start the clock, because nothing is going to
stop it in the future, and it messes the rest of the view.

https://bugzilla.gnome.org/show_bug.cgi?id=735556