* xselect.c: Integer signedness and overflow fixes.

* xselect.c: Integer signedness and overflow fixes.

(Fx_register_dnd_atom, x_handle_dnd_message):
Use ptrdiff_t, not size_t, since we prefer signed.
(Fx_register_dnd_atom): Check for ptrdiff_t (and size_t) overflow.
* xterm.h (struct x_display_info): Use ptrdiff_t, not size_t, for
x_dnd_atoms_size and x_dnd_atoms_length.