avformat/matroskaenc: Check BlockAdditional size before use

Don't read a 64bit number before having checked that the data is at
least 8 bytes long.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/matroskaenc.c

@@ -2142,10 +2142,14 @@ static int mkv_write_block(AVFormatContext *s, AVIOContext *pb,
                                         AV_PKT_DATA_MATROSKA_BLOCKADDITIONAL,
                                         &side_data_size);
     if (side_data) {
+        if (side_data_size < 8) {
+            side_data_size = 0;
+        } else {
             additional_id   = AV_RB64(side_data);
             side_data      += 8;
             side_data_size -= 8;
         }
+    }
 
     if ((side_data_size && additional_id == 1) || discard_padding) {
         block_group = start_ebml_master(pb, MATROSKA_ID_BLOCKGROUP, 0);