avcodec/vp8: Check buffer size in vp8_decode_frame_header()

avoids null pointer dereference
Fixes: signal_sigsegv_d5de40_964_vp80-00-comprehensive-010.ivf with memlimit of 1048576

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vp8.c

@@ -639,6 +639,11 @@ static int vp8_decode_frame_header(VP8Context *s, const uint8_t *buf, int buf_si
     int width  = s->avctx->width;
     int height = s->avctx->height;
 
+    if (buf_size < 3) {
+        av_log(s->avctx, AV_LOG_ERROR, "Insufficent data (%d) for header\n", buf_size);
+        return AVERROR_INVALIDDATA;
+    }
+
     s->keyframe  = !(buf[0] & 1);
     s->profile   =  (buf[0]>>1) & 7;
     s->invisible = !(buf[0] & 0x10);