atrac3: avoid oversized shifting in decode_bytes()

When `off' is 0, `0x537F6103 << 32' in the following expression invokes
undefined behavior, the result of which is not necessarily 0.

    (0x537F6103 >> (off * 8)) | (0x537F6103 << (32 - (off * 8)))

Avoid oversized shifting.

CC: libav-stable@libav.org

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
