Use ptrdiff_t, not int, for overlay counts.

Use ptrdiff_t, not int, for overlay counts.

* buffer.h (overlays_at, sort_overlays, GET_OVERLAYS_AT):
* editfns.c (overlays_around, get_pos_property):
* textprop.c (get_char_property_and_overlay):
* xdisp.c (next_overlay_change, note_mouse_highlight):
* xfaces.c (face_at_buffer_position):
* buffer.c (overlays_at, sort_overlays, Foverlays_at)
(Fnext_overlay_change, Fprevious_overlay_change):
Use ptrdiff_t, not int, for sizes.
(overlays_at): Check for size-calculation overflow.