Add null check for rtsp source

During playback contents with multi audio tracks or multi video tracks,
crash occurs in checkBuffering() which is added from Android-N.
The reason is because instance is not set in mSource in onConnected().
This commit adds null check to prevent crash.

Bug: 29520487

Change-Id: I893c11a7339e938b1f3b79805c2fb3d9be5cfc96
Signed-off-by: ZHU XINGYUE <zhu.x.xingyue@sonymobile.com>