Fix concat seeking SEEK_END case.

Patch by Wolfram Gloger wmglo ^ dent.med.uni-muenchen.de.

Originally committed as revision 22306 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/concat.c

@@ -160,9 +160,9 @@ static int64_t concat_seek(URLContext *h, int64_t pos, int whence)
     switch (whence) {
     case SEEK_END:
         for (i = data->length - 1;
-             i && pos < -nodes[i-1].size;
+             i && pos < -nodes[i].size;
              i--)
-            pos += nodes[i-1].size;
+            pos += nodes[i].size;
         break;
     case SEEK_CUR:
         /* get the absolute position */