Fire intro.oak_speech.finished only once (#308) (#309)

The shrink timeline called finish() every frame once past its end.
finish() emits finished and then pops the stack, so a listener that
pushes a screen (a warp, a menu) had that screen popped in the speech's
place: the speech stayed alive and re-fired the event every frame,
repeating the listener's side effects each time. Clear the shrink
state before finishing so the timeline cannot run finish() twice.

Co-authored-by: johnjohto <johnjohto@users.noreply.github.com>

src/ui/OakSpeech.lua

@@ -549,6 +549,10 @@ function OakSpeech:update(dt)
   elseif s.frame >= 79 and s.frame <= 102 then
     self.fadeLevel = math.floor((s.frame - 79) / 8) + 1
   elseif s.frame > 102 then
+    -- clear before finish(): a finished-listener that pushes a state gets
+    -- ITS state popped in the speech's place, and a live shrink would call
+    -- finish() again next frame, re-firing the event every frame (#308)
+    self.shrink = nil
     self:finish()
   end
 end