Prevent DAUD PCM encoder from fetching values outside of ff_reverse[] array bounds when input sample values are < 0.

Originally committed as revision 14658 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/pcm.c

@@ -209,7 +209,7 @@ static int pcm_encode_frame(AVCodecContext *avctx,
         break;
     case CODEC_ID_PCM_S24DAUD:
         for(;n>0;n--) {
-            uint32_t tmp = ff_reverse[*samples >> 8] +
+            uint32_t tmp = ff_reverse[(*samples >> 8) & 0xff] +
                            (ff_reverse[*samples & 0xff] << 8);
             tmp <<= 4; // sync flags would go here
             bytestream_put_be24(&dst, tmp);