ao_null: fix simulated buffer size

The size accidentally defaulted to 200 seconds instead of 200
milliseconds, which had fatal consequences when trying to use it.

audio/out/ao_null.c

@@ -182,7 +182,7 @@ const struct ao_driver audio_out_null = {
     .resume    = resume,
     .priv_size = sizeof(struct priv),
     .priv_defaults = &(const struct priv) {
-        .bufferlen = 200,
+        .bufferlen = 0.2,
         .outburst = 256,
     },
     .options = (const struct m_option[]) {