Fix issue where video player is not released by preview fragment.

app/src/main/java/org/thoughtcrime/securesms/mediapreview/VideoMediaPreviewFragment.java

@@ -89,6 +89,12 @@ public final class VideoMediaPreviewFragment extends MediaPreviewFragment {
     return itemView;
   }
 
+  @Override
+  public void onDestroyView() {
+    super.onDestroyView();
+    cleanUp();
+  }
+
   @Override
   public void cleanUp() {
     if (videoView != null) {