Fix subtle problems with linum-mode and invisible text

Fix subtle problems with linum-mode and invisible text

 src/xdisp.c (handle_invisible_prop): Fix up it->position even when
 we are going to load overlays at the beginning of the invisible
 text.
 (setup_for_ellipsis): Reset the ignore_overlay_strings_at_pos_p
 flag also here.
 (next_overlay_string): Set the overlay_strings_at_end_processed_p
 flag only if the overlays just processed were actually loaded at
 EOB.