added validity checking

libao.rkt

@@ -16,6 +16,7 @@
          ao-bufsize-async
          ao-clear-async
          ao-pause
+         ao-valid? 
          )
 
 (define device-number 1)
@@ -71,6 +72,9 @@
     )
   )
 
+(define (ao-valid? handle)
+  (not (eq? (ao-handle-async-player handle) #f)))
+
 (define (ao-play handle at-time-in-s music-duration-s buffer buf-len buf-type)
   (let* ((bytes-per-sample (ao-handle-bytes-per-sample handle))
          (bits (ao-handle-bits handle))
@@ -79,6 +83,8 @@
          (endianess (ao-handle-byte-format handle))
          (buf-info (ffi:make-BufferInfo_t buf-type bits rate channels endianess))
          )
+    (unless (ao-valid? handle)
+      (error "Cannot play on an invalid ao-device"))
     (ffi:ao_play_async (ao-handle-async-player handle)
                        (exact->inexact at-time-in-s)
                        (exact->inexact music-duration-s)