asserts to check if assumed conditions really are true

Originally committed as revision 5143 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/motion_est.c

@@ -901,6 +901,8 @@ static int interlaced_search(MpegEncContext *s, int ref_index,
             int16_t (*mv_table)[2]= mv_tables[block][field_select];
 
             if(user_field_select){
+                assert(field_select==0 || field_select==1);
+                assert(field_select_tables[block][xy]==0 || field_select_tables[block][xy]==1);
                 if(field_select_tables[block][xy] != field_select)
                     continue;
             }