dca: fix misaligned access in ff_dca_convert_bitstream

The function is used on unaligned buffers (such as those provided
by AVPacket), accessing them as uint16_t causes SIGBUS crashes on
architectures like SPARC.

This fixes ubsan runtime error: load of misaligned address for type
'const uint16_t', which requires 2 byte alignment

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
