libavcodec: Don't crash in avcodec_encode_audio if time_base isn't set

Earlier, calling avcodec_encode_audio worked fine even if time_base
wasn't set. Now it crashes due to trying to scale the output pts to
the codec context time base. This affects e.g. VLC.

If no time_base is set for audio codecs, set it to the sample
rate.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 9a7dc618c5)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavcodec/utils.c

@@ -744,6 +744,12 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, AVCodec *codec, AVD
            avctx->error_recognition, avctx->err_recognition);
 #endif
 
+    if (avctx->codec_type == AVMEDIA_TYPE_AUDIO &&
+        (!avctx->time_base.num || !avctx->time_base.den)) {
+        avctx->time_base.num = 1;
+        avctx->time_base.den = avctx->sample_rate;
+    }
+
     if (HAVE_THREADS && !avctx->thread_opaque) {
         ret = ff_thread_init(avctx);
         if (ret < 0) {