alsdec: only adapt order for positive max_order

For max_order = 0 the clipping range is invalid. (amin = 2, amax = 1)

CC: libav-stable@libav.org
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>
