[Bug] Fix Valgrind reports memory corruption when Setting the Mute prope...

...rty

- Valgrind reports memory corruption when Setting the Mute property.
- If you set the Mute value, the received last change event only contains
a value for mute. It does not contain a value for Volume, and so
device_volume was not initialized.
- The fix consists in initializing device_volume with G_MAXUINT, so we can
check if the volume was part of the received last change event.

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>