This is required to use it as an AVCodec.supported_samplerates array.
Adding the sentinel has been forgotten in 4679a474.
Without it e.g. the FATE-test ffmpeg-filter_complex_audio fails with ASAN.
Reviewed-by: Carl Eugen Hoyos <ceffmpeg@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>