matroskadec: Pad AAC extradata.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

CC: libav-stable@libav.org

libavformat/matroskadec.c

@@ -1440,7 +1440,7 @@ static int matroska_read_header(AVFormatContext *s, AVFormatParameters *ap)
         } else if (codec_id == CODEC_ID_AAC && !track->codec_priv.size) {
             int profile = matroska_aac_profile(track->codec_id);
             int sri = matroska_aac_sri(track->audio.samplerate);
-            extradata = av_malloc(5);
+            extradata = av_mallocz(5 + FF_INPUT_BUFFER_PADDING_SIZE);
             if (extradata == NULL)
                 return AVERROR(ENOMEM);
             extradata[0] = (profile << 3) | ((sri&0x0E) >> 1);