Clarify some assert usage.

They are not to check application supplied parameters.

Originally committed as revision 17278 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo_xvmc.c

@@ -285,6 +285,8 @@ void ff_xvmc_decode_mb(MpegEncContext *s)
 
     assert(render->filled_mv_blocks_num     <= render->total_number_of_mv_blocks);
     assert(render->next_free_data_block_num <= render->total_number_of_data_blocks);
+    /*The above conditions should not be able to fail as long as this function is used
+    and following if() automatically call callback to free blocks. */
 
 
     if (render->filled_mv_blocks_num >= render->total_number_of_mv_blocks)