dca: fix misaligned access in avpriv_dca_convert_bitstream

src and dst are only 8-bit-aligned, so 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

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
