twn_audio.c: fix freeing on unnamed channels

src/twn_audio.c

@@ -556,7 +556,7 @@ void audio_callback(void *userdata, uint8_t *stream, int len) {
 
     /* ditch finished unnamed */
     size_t i = 0;
-    while (i < unnamed_audio_channels_len) {
+    while (i < arrlenu(ctx.unnamed_audio_channels)) {
         if (ctx.unnamed_audio_channels[i].finished) {
             free_audio_channel(ctx.unnamed_audio_channels[i]);
             arrdelswap(ctx.unnamed_audio_channels, i);