smacker: error out if palette copy-with-offset overruns palette size.

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

libavformat/smacker.c

@@ -265,8 +265,15 @@ static int smacker_read_packet(AVFormatContext *s, AVPacket *pkt)
                     sz += (t & 0x7F) + 1;
                     pal += ((t & 0x7F) + 1) * 3;
                 } else if(t & 0x40){ /* copy with offset */
-                    off = avio_r8(s->pb) * 3;
+                    off = avio_r8(s->pb);
                     j = (t & 0x3F) + 1;
+                    if (off + j > 0xff) {
+                        av_log(s, AV_LOG_ERROR,
+                               "Invalid palette update, offset=%d length=%d extends beyond palette size\n",
+                               off, j);
+                        return AVERROR_INVALIDDATA;
+                    }
+                    off *= 3;
                     while(j-- && sz < 256) {
                         *pal++ = oldpal[off + 0];
                         *pal++ = oldpal[off + 1];