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.

audio_ao.c

@@ -55,7 +55,7 @@ static int init(int argc, char **argv) {
  switch (opt) {
  case 'd':
  driver = ao_driver_id(optarg);
- if (!driver)
+ if (driver < 0)
  die("could not find ao driver %s", optarg);
  break;
  case 'i':