encode: restore audio muxer timebase use

Seems to crash hard if an error happens somewhere at init. Who cares.

Part of #7524.

audio/out/ao_lavc.c

@@ -87,6 +87,9 @@ static void select_format(struct ao *ao, const AVCodec *codec)
 static void on_ready(void *ptr)
 {
     struct ao *ao = ptr;
+    struct priv *ac = ao->priv;
+
+    ac->worst_time_base = encoder_get_mux_timebase_unlocked(ac->enc);
 
     ao_add_events(ao, AO_EVENT_INITIAL_UNBLOCK);
 }

common/encode_lavc.c

@@ -491,6 +491,11 @@ done:
         av_packet_unref(pkt);
 }
 
+AVRational encoder_get_mux_timebase_unlocked(struct encoder_context *p)
+{
+    return p->mux_stream->st->time_base;
+}
+
 void encode_lavc_discontinuity(struct encode_lavc_context *ctx)
 {
     if (!ctx)

common/encode_lavc.h

@@ -111,6 +111,10 @@ bool encoder_init_codec_and_muxer(struct encoder_context *p,
 // Encode the frame and write the packet. frame is ref'ed as need.
 bool encoder_encode(struct encoder_context *p, AVFrame *frame);
 
+// Return muxer timebase (only available after on_ready() has been called).
+// Caller needs to acquire encode_lavc_context.lock (or call it from on_ready).
+AVRational encoder_get_mux_timebase_unlocked(struct encoder_context *p);
+
 double encoder_get_offset(struct encoder_context *p);
 
 #endif