Fix concat seek result.

Patch by Wolfram Gloger wmglo AT-SIGN dent.med.uni-muenchen DOT de.

Originally committed as revision 22302 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/concat.c

@@ -183,7 +183,7 @@ static int64_t concat_seek(URLContext *h, int64_t pos, int whence)
     if (result >= 0) {
         data->current = i;
         while (i)
-            result += nodes[i--].size;
+            result += nodes[--i].size;
     }
     return result;
 }