avformat/mov: clear padding area in mov_read_extradata()

Fixes: msan_uninit-mem_7f58816b71e3_7025_mov_svq3___svq3_weird_prediction2.mov
Fixes use of uninitialized memory
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -1010,6 +1010,7 @@ static int mov_read_extradata(MOVContext *c, AVIOContext *pb, MOVAtom atom,
         av_log(c->fc, AV_LOG_WARNING, "truncated extradata\n");
         st->codec->extradata_size -= atom.size - err;
     }
+    memset(buf + 8 + err, 0, FF_INPUT_BUFFER_PADDING_SIZE);
     return 0;
 }