Fix B producer off by one frame in mlt_playlist_mix_out().
Fix B producer off by one frame in mlt_playlist_mix_out().

This fixes Shotcut bug https://github.com/mltframework/shotcut/issues/
407

j-b-m/mlt,siddharudh/mlt,mltframework/mlt,siddharudh/mlt,j-b-m/mlt,j-b-m/mlt,siddharudh/mlt,j-b-m/mlt,mltframework/mlt,siddharudh/mlt,mltframework/mlt,mltframework/mlt,siddharudh/mlt,siddharudh/mlt,mltframework/mlt,j-b-m/mlt,siddharudh/mlt,j-b-m/mlt,mltframework/mlt,siddharudh/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,siddharudh/mlt,mltframework/mlt,j-b-m/mlt