Fix potentially unaligned accesses in ff_copy_bits()

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
type modifiers.

Originally committed as revision 19318 to svn://svn.ffmpeg.org/ffmpeg/trunk