Checks result of ao-open-live function

[y2q-actionman]

I am trying to use your mixalot on libao environment.

When I called create-mixer with unavailable sample rate like -1, I saw messages "libao error." are written endlessly.

y2q-no-MacBook-Air-3:mixalot$ sbcl
This is SBCL 2.1.3, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
* (ql:quickload :mixalot)
To load "mixalot":
  Load 1 ASDF system:
    mixalot
; Loading "mixalot"
..................................................
[package mixalot].............
(:MIXALOT)
* (mixalot:main-thread-init)
* (mixalot:create-mixer :rate -1)
ao_macosx ERROR: AudioUnitSetProperty error => -10868
#S(MIXALOT:MIXER
   :STREAM-LOCK #<SB-THREAD:MUTEX "Mixer lock" (free)>
   :STREAM-LIST NIL
   :CURRENT-TIME 0
   :RATE -1
   :SHUTDOWN-FLAG NIL
   :STREAM-STATE #<HASH-TABLE :TEST EQL :COUNT 0 {10049536E3}>
   :DEVICE #.(SB-SYS:INT-SAP #X00000000))
* libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao error.libao erro

(...omitted)

I think you should check the result of ao-open-live function.
This pull request adds the check, and throws simple-error if failed.

(We should see errno for handling the error more properly.
But this PR does not do so because it requires CFFI wrapper, like osicat library does. It is hard work for me...)