Added missing fireball's TTL.

src/content_mob.cpp

@@ -450,6 +450,7 @@ void content_mob_init()
 	f->description = wgettext("Fireball");
 	f->level = MOB_DESTRUCTIVE;
 	f->setTexture("mob_fireball.png");
+    f->lifetime = 20.0;
 	f->punch_action = MPA_IGNORE;
 	f->motion = MM_CONSTANT;
 	f->motion_type = MMT_FLY;