[ASVideoNode] Time observer fix (TextureGroup#604)

* [ASVideoNode] Move time observer handling to player observers methods.

* Update CHANGELOG.md.

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## master
 
 * Add your own contributions to the next release on the line below this with your name.
+- [ASVideoNode] Fix unreleased time observer. [Flo Vouin](https://github.com/flovouin)
 - [PINCache] Set a default .byteLimit to reduce disk usage and startup time. [#595](https://github.com/TextureGroup/Texture/pull/595) [Scott Goodson](https://github.com/appleguy)
 - [ASNetworkImageNode] Fix deadlock in GIF handling. [#582](https://github.com/TextureGroup/Texture/pull/582) [Garrett Moon](https://github.com/garrettmoon)
 - [ASDisplayNode] Add attributed versions of a11y label, hint and value. [#554](https://github.com/TextureGroup/Texture/pull/554) [Alexander Hüllmandel](https://github.com/fruitcoder)

Source/ASVideoNode.mm

@@ -181,12 +181,6 @@ - (void)prepareToPlayAsset:(AVAsset *)asset withKeys:(NSArray<NSString *> *)requ
   if (self.image == nil && self.URL == nil) {
     [self generatePlaceholderImage];
   }
-
-  __weak __typeof(self) weakSelf = self;
-  _timeObserverInterval = CMTimeMake(1, _periodicTimeObserverTimescale);
-  _timeObserver = [_player addPeriodicTimeObserverForInterval:_timeObserverInterval queue:NULL usingBlock:^(CMTime time){
-    [weakSelf periodicTimeObserver:time];
-  }];
 }
 
 - (void)addPlayerItemObservers:(AVPlayerItem *)playerItem
@@ -231,10 +225,21 @@ - (void)addPlayerObservers:(AVPlayer *)player
   }
 
   [player addObserver:self forKeyPath:kRate options:NSKeyValueObservingOptionNew context:ASVideoNodeContext];
+
+  __weak __typeof(self) weakSelf = self;
+  _timeObserverInterval = CMTimeMake(1, _periodicTimeObserverTimescale);
+  _timeObserver = [player addPeriodicTimeObserverForInterval:_timeObserverInterval queue:NULL usingBlock:^(CMTime time){
+    [weakSelf periodicTimeObserver:time];
+  }];
 }
 
 - (void) removePlayerObservers:(AVPlayer *)player
 {
+  if (_timeObserver != nil) {
+    [player removeTimeObserver:_timeObserver];
+    _timeObserver = nil;
+  }
+
   @try {
     [player removeObserver:self forKeyPath:kRate context:ASVideoNodeContext];
   }
@@ -836,8 +841,6 @@ - (void)setShouldBePlaying:(BOOL)shouldBePlaying
 
 - (void)dealloc
 {
-  [_player removeTimeObserver:_timeObserver];
-  _timeObserver = nil;
   [self removePlayerItemObservers:_currentPlayerItem];
   [self removePlayerObservers:_player];