Fix underflow bug on linux

src/alsa/mod.rs

@@ -302,7 +302,7 @@ impl Voice {
             let available = unsafe { alsa::snd_pcm_avail(*channel) };
 
             if available == -32 {
-                0       // buffer underrun
+                self.buffer_len as alsa::snd_pcm_sframes_t       // buffer underrun
             } else if available < 0 {
                 check_errors(available as libc::c_int).expect("could not write to buffer");
                 unreachable!()