Warn if volume and/or muted state is inconsistent (as we only expose a
single volume/muted state).
Also, don't query for current volume if max == 0 (i.e. typically a
digital output).
Finally, make absolutely sure that volume min is really less (or
equal) to volume max, and that the current volume level is between min
and max.
When snd_mixer_selem_get_*() fails, the values were left
uninitialized, trigger a valgrind warning.
In addition to fixing this, the error(s) are now logged (but only as
warnings, as they are not fatal).
