adpcm: Only increment heap_pos after finding a good enough sample

This increases the PSNR slightly (about 0.1 dB) for trellis sizes
below 8, and gives equal PSNR for sizes above that.

Originally committed as revision 25769 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/adpcm.c

@@ -401,9 +401,10 @@ static void adpcm_compress_trellis(AVCodecContext *avctx, const short *samples,
                     } else {\
                         /* Try to replace one of the leaf nodes with the new \
                          * one, but try a different slot each time. */\
-                        pos = (frontier >> 1) + (heap_pos++ & ((frontier >> 1) - 1));\
+                        pos = (frontier >> 1) + (heap_pos & ((frontier >> 1) - 1));\
                         if (ssd > nodes_next[pos]->ssd)\
                             goto next_##NAME;\
+                        heap_pos++;\
                     }\
                     *h = generation;\
                     u = nodes_next[pos];\