Fixed the Music app crash issue when the repeat button is disabled.

When the music application is set to REPEAT_NONE mode, the mNextMediaPlayer
didn't set to null. Therefore, the music would play next song, but the index
is invaild and causes the app crash.

Change-Id: I991ab50a05636ad3e14ea826cf9a43af7dacdb0a

Signed-off-by: Jun Tian <jun.j.tian@intel.com>
Signed-off-by: Zhu Yanglin <yanglinx.zhu@intel.com>