fix possible NPE

We register ourselfs as a callback receiver BEFORE the adapter has been created.
This crash is unlikely but happens once in a while according to the play store.

src/ch/blinkenlights/android/vanilla/LibraryActivity.java

@@ -862,7 +862,8 @@ public class LibraryActivity
 	@Override
 	public void onMediaChange()
 	{
-		mPagerAdapter.invalidateData();
+		if (mPagerAdapter != null)
+			mPagerAdapter.invalidateData();
 	}
 
 	@Override