producer_avformat.c: improve audio synchronization after seek (including in point)

[melted] / profiles / cvd_ntsc

description=CVD NTSC
frame_rate_num=30000
frame_rate_den=1001
width=352
height=480
progressive=0
sample_aspect_num=20
sample_aspect_den=11
display_aspect_num=4
display_aspect_den=3