Fix iOS VideoPlayer memory leak.

# Before
* `MPMoviePlayerController alloc] init~~` -> retain count equal 1.
* set to `strong` property  -> retain count to 2.
* set nullptr to `self.moviePlayer` -> retain count to 1.
* **Leak!**

# After
* `MPMoviePlayerController alloc] init~~` -> retain count equal 1.
* call `autorelease` -> retain count equal 0 when out of autoReleasePool.
* set to `strong` property  -> retain count to 1.
* set nullptr to `self.moviePlayer` -> retain count to 0.
* **MemFree**