Eleven: add null-check while scrolling fading fragment to avoid crash

By inspection, it appears getContainingActivity() might return null while
the fragment is being destroyed. Added a null check to prevent a crash in
this eventuality.

https://cyanogen.atlassian.net/browse/MUSIC-129

Change-Id: I534e427f1e0b04a7acd3cca1632717a2b06f4923