truemotion1: Check input buffer size against header size.

Fixes overread.

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

libavcodec/truemotion1.c

@@ -314,7 +314,7 @@ static int truemotion1_decode_header(TrueMotion1Context *s)
     const uint8_t *sel_vector_table;
 
     header.header_size = ((s->buf[0] >> 5) | (s->buf[0] << 3)) & 0x7f;
-    if (s->buf[0] < 0x10)
+    if (s->buf[0] < 0x10 || header.header_size >= s->size)
     {
         av_log(s->avctx, AV_LOG_ERROR, "invalid header size (%d)\n", s->buf[0]);
         return -1;