Fix regression with volumecontroller

[ashthespy]

I seem to have introduced issues with the software mixer with dd3f21a, sorry!
This is just fixing the symptoms, as I believe the linting has unearthed the real error which was just being silently ignored.
The error comes from these comparisons:

         case 'mute':
           // Mute
           self.getVolume(function (err, vol) {
-            if (vol == null) {
+            if (err) {
+              self.logger.error('Cannot get ALSA Volume: ' + err);
+            }
+            if (vol === null) {

The problem was that vol was never null when the parsing failed, but instead was undefined.
Thus undefined == null => true but undefined === null => false

Now this brings to light the actual underlying issue.

Both the regular expressions, fails to parse software mixer output, which doesn't contain Playback

  Simple mixer control 'SoftMaster',0
  Capabilities: volume
  Playback channels: Front Left - Front Right
  Capture channels: Front Left - Front Right
  Limits: 0 - 99
  Front Left: 10 [10%]
  Front Right: 10 [10%]
# Compared to Hardware mixer
  Simple mixer control 'PCM',0
  Capabilities: pvolume pswitch pswitch-joined
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 110
  Mono:
  Front Left: Playback 0 [0%] [-55.00dB] [on]
  Front Right: Playback 0 [0%] [-55.00dB] [on]

If you give me a few samples of different mixer outputs, we can write some tests as well..

[volumio]

Perfect! Thanks!