h264: limit allowed pred modes in ff_h264_check_intra_pred_mode() to 3

Higher modes are not allowed for 16x16/chroma, which is what this
function is used for. Otherwise this function would return 0 (vertical
prediction) for invalid higher modes, which could result in invalid
reads.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org

libavcodec/h264.c

@@ -534,10 +534,10 @@ int ff_h264_check_intra4x4_pred_mode(H264Context *h)
  */
 int ff_h264_check_intra_pred_mode(H264Context *h, int mode, int is_chroma)
 {
-    static const int8_t top[7]  = { LEFT_DC_PRED8x8, 1, -1, -1 };
-    static const int8_t left[7] = { TOP_DC_PRED8x8, -1, 2, -1, DC_128_PRED8x8 };
+    static const int8_t top[4]  = { LEFT_DC_PRED8x8, 1, -1, -1 };
+    static const int8_t left[5] = { TOP_DC_PRED8x8, -1,  2, -1, DC_128_PRED8x8 };
 
-    if (mode > 6U) {
+    if (mode > 3U) {
         av_log(h->avctx, AV_LOG_ERROR,
                "out of range intra chroma pred mode at %d %d\n",
                h->mb_x, h->mb_y);