utf8.c: Don't copy a buffer to itself

utf8.c: Don't copy a buffer to itself

memcpy(), which is what Copy() resolves to, is not supposed to handle
the possibility of overlapping source and destination.  In some cases
in this code, the source and destination pointers are identical.  What
should happen then is a no-op, so just don't do the copy in that case.
If the ptrs aren't identical, they won't otherwise overlap, so the
Copy() is valid except for when they are identical.