matroskadec: reset size when freeing data.

Fixes null pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -770,6 +770,7 @@ static int ebml_read_binary(AVIOContext *pb, int length, EbmlBin *bin)
     bin->pos  = avio_tell(pb);
     if (avio_read(pb, bin->data, length) != length) {
         av_freep(&bin->data);
+        bin->size = 0;
         return AVERROR(EIO);
     }