sound: clear the value on level_bar when stream is empty

We met an Input Device level_bar display issue on a machine which has no internal mic. At first there is no external mic plugged, so the Input Device list is empty and level_bar is gray color, after we plug an external mic, the level_bar has red color ripples, then we unplug the external mic, the Input Device list changes to empty and we expect the level_bar changes back to gray color, but some bars are still red color.

Here clear the self->value to 0 if the stream is empty, then the level_bar will change back to gray color when Input/Output device list is empty.

Signed-off-by: Hui Wang hui.wang@canonical.com