afifo: fix request_samples on the last frame in certain cases

The current code can fail to return the last frame if it contains
exactly the requested number of samples.

Fixes the join filter test, which previously did not include the last
408 samples in most cases.

CC:libav-stable@libav.org

Signed-off-by: Diego Biurrun <diego@biurrun.de>
(cherry picked from commit 9bfc6e02bae9de354fb9ba09a8a140e83eeadf7d)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

Conflicts:
	libavfilter/fifo.c
	tests/fate/filter-audio.mak
hls, segment: fix splitting for audio-only streams.

CC:libav-stable@libav.org
(cherry picked from commit cf679b9476727a237c8006c685ace18acba149ab)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>
Prepare for 9.6 Release