[BUG] Flappers both exit *and* explode

[WillLem]

Scenario (describes a trick used in some levels, hence the spoiler tag)

A digger that is digging directly above an exit's trigger is assigned a Bomber to stop them mid-dig, thus not digging all the way through the terrain, and themselves dropping directly towards the exit.

If close enough to the trigger, the lemming will exit before exploding. However, the explosion subsequently happens anyway and the lemming does not count as having been saved.

Expected behaviour: Flapper lemming has exited - the explosion should not happen, and the lemming should count towards the save requirement.