chromeos: Check ALSA return values in mixer code.

chromeos: Check ALSA return values in mixer code.

We're seeing a NaN value sometimes get saved (incorrectly)
to the prefs file in the volume setting, which results in
the prefs file being unparseable the next time Chrome
starts, which results in us going through OOBE again.  The
prefs code ought to write NaNs correctly so that they don't
corrupt the file, but in the meantime, this change checks
the return values from a bunch of ALSA functions (my best
theory as to how the NaN is creeping in) and for good
measure, also maps NaN volume values to the minimum volume.

BUG=chromium-os:12229
TEST=built and ran it (i haven't been able to repro the problem)

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=75698

[davejcool, 2011-02-23 02:35:50]

LGTM.

If nothing else, the extensive checking of alsa return values should shed light
on what is causing the -Nan value.