countdown: handles out of range temperature index.

In system status, some temerature value may not exists. We shall
handle IndexError gracefully.

BUG=chrome-os-partner:20679
TEST=Alter countdown dargs's temp_criteria by giving a large
  index to observe the warning 'Temperature unavailable'.

Change-Id: I192900b8034a6cd0fef960f1dc67e6f366ddebbb
Reviewed-on: https://gerrit.chromium.org/gerrit/61395
Commit-Queue: Shuo-Peng Liao <deanliao@chromium.org>
Reviewed-by: Shuo-Peng Liao <deanliao@chromium.org>
Tested-by: Shuo-Peng Liao <deanliao@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/61562
Reviewed-by: Bowgo Tsai <bowgotsai@chromium.org>
1 file changed