avformat/oggparsetheora: zero extradata padding area

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f66fa0ac09d_9197_theora_a4_v6_k250_s0.ogg
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/oggparsetheora.c

@@ -131,6 +131,8 @@ static int theora_header(AVFormatContext *s, int idx)
         st->codec->extradata_size = 0;
         return err;
     }
+    memset(st->codec->extradata + cds, 0, FF_INPUT_BUFFER_PADDING_SIZE);
+
     cdp    = st->codec->extradata + st->codec->extradata_size;
     *cdp++ = os->psize >> 8;
     *cdp++ = os->psize & 0xff;