fixing alt_scan for the first frame (variable was reset)

Originally committed as revision 1045 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h263dec.c

@@ -334,7 +334,7 @@ uint64_t time= rdtsc();
     s->bitstream_buffer_size=0;
 
     if (!s->context_initialized) {
-        if (DCT_common_init(s) < 0) //we need the idct permutaton for reading a custom matrix
+        if (MPV_common_init(s) < 0) //we need the idct permutaton for reading a custom matrix
             return -1;
     }