wmv1: check that the input buffer is large enough

Fixes null ptr deref
Fixes Ticket1367

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit f23a2418fb)

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

libavcodec/wnv1.c

@@ -67,6 +67,11 @@ static int decode_frame(AVCodecContext *avctx,
     int prev_y = 0, prev_u = 0, prev_v = 0;
     uint8_t *rbuf;
 
+    if(buf_size<=8) {
+        av_log(avctx, AV_LOG_ERROR, "buf_size %d is too small\n", buf_size);
+        return AVERROR_INVALIDDATA;
+    }
+
     rbuf = av_malloc(buf_size + FF_INPUT_BUFFER_PADDING_SIZE);
     if(!rbuf){
         av_log(avctx, AV_LOG_ERROR, "Cannot allocate temporary buffer\n");