The logic did not follow the documented behaviour and that caused skipping of
some audio in the loop and in the leftover buffer.
Example command line which should produce a smooth sine wave for the whole
duration of the output:
ffmpeg -f lavfi -i "sine=r=48000:f=480:d=4" -af "aloop=loop=4:start=48000:size=48000" out.wav
Fixes ticket #11283.
Signed-off-by: Marton Balint <cus@passwd.hu>