Fix libao output driver.

The driver id returned by libao wasn't checked correctly.
When the driver did not exist, libao returns a negative number. The
libao output driver checked for a non zero id.
This caused shairport to refuse to use the first driver, and not fail
when using a non-existant driver.
Merge pull request #224 from plietar/fix-libao-driver-check

Fix libao output driver.