avformat/bit: check that pkt->size is 10 in write_packet

Ohter packet sizes are not supported by this muxer.

This avoids a null pointer dereference of pkt->data.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit eeda2c3de8)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/bit.c

@@ -133,6 +133,9 @@ static int write_packet(AVFormatContext *s, AVPacket *pkt)
     GetBitContext gb;
     int i;
 
+    if (pkt->size != 10)
+        return AVERROR(EINVAL);
+
     avio_wl16(pb, SYNC_WORD);
     avio_wl16(pb, 8 * 10);