h264_cavlc: check the size of the intra PCM data.

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

libavcodec/h264_cavlc.c

@@ -765,6 +765,10 @@ decode_intra_mb:
 
         // We assume these blocks are very rare so we do not optimize it.
         h->intra_pcm_ptr = align_get_bits(&h->gb);
+        if (get_bits_left(&h->gb) < mb_size) {
+            av_log(h->avctx, AV_LOG_ERROR, "Not enough data for an intra PCM block.\n");
+            return AVERROR_INVALIDDATA;
+        }
         skip_bits_long(&h->gb, mb_size);
 
         // In deblocking, the quantizer is 0