Improve resilience of volume change.

Building on the previous fix for the audio volume jumps experienced
when switching tracks, this adds a double check that the underlying audio
layer has actually set the volume to the new value. This may fail because
the COMXAudio object it not yet initialised.

If there is a failure, then the volume change is retried in the next
iteration of COMXPlayer::Process().
Merge pull request #2909 from sgebbie/master

[rbp/omxplayer] Volume Jump Fix