Fixed not being able to complete missions after destroying interceptions.

There was a check that was supposed to prevent completing missions on
death, but it also prevented completing missions after an area was
finished. This was in particular a problem in interceptions, where
rescuing slaves after destroying interceptions didn't have any effect
on the slave rescue mission.

src/missions.cpp

@@ -390,7 +390,7 @@ static void evaluateRequirement(int type, int id, int *completed, int *targetVal
 void updateMissionRequirements(int type, int id, int value)
 {
 	// Can't complete missions if you're dead!
-	if (engine.missionCompleteTimer != 0)
+	if (player.shield <= 0)
 		return;
 
 	char message[25];