dca: Validate the lfe parameter

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit a9d50bb578ec04c085a25f1e023f75e0e4499d5e)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>