The sample rate index is 3 bits even if currently index 5, 6 and 7 are
not supported.
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
(cherry picked from commit 0933fd1533560fbc718026e12f19a4824b041237)
Signed-off-by: Reinhard Tartler <siretart@tauware.de>