Fix potentially unaligned accesses in ff_copy_bits()
A pointer should never be assigned a value which may have less than the
required alignment of the target type. Compilers may assume pointer
values have the required alignment, and emit normal load/store instructions.
Unaligned pointers should use a character type or compiler-specific
Originally committed as revision 19318 to svn://svn.ffmpeg.org/ffmpeg/trunk