avformat/flvdec: initialize context before reading from it

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f9b8387069e_5377_flv_with_pcm_s16be_audio_track.flv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/flvdec.c

@@ -916,7 +916,7 @@ retry_duration:
             flv->last_channels    =
             channels              = st->codec->channels;
         } else {
-            AVCodecContext ctx;
+            AVCodecContext ctx = {0};
             ctx.sample_rate = sample_rate;
             flv_set_audio_codec(s, st, &ctx, flags & FLV_AUDIO_CODECID_MASK);
             sample_rate = ctx.sample_rate;