CRAS: rclient - Check aud_fd arg first.
CRAS: rclient - Check aud_fd arg first.

The check for aud_fd being valid can be done at the beginning of the
add stream function.  Doing this here will fail earlier and make it
easier to add retries for bad iodevs.

BUG=chromium-os:37798
TEST=make check

Change-Id: Ia06eacb9242f7f112d3d75d6acab3f66995b97e6
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/41169

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