Fix undefined behavior when fetching glyphs from the display vector.

Fix undefined behavior when fetching glyphs from the display vector.

You can trigger this rather obscure bug by enabling selective display
if the second glyph in its display vector has an invalid face.  For
example, evaluate

(set-display-table-slot standard-display-table
                        'selective-display [?A (?B . invalid)])

and then enable selective display.

* src/xdisp.c (next_element_from_display_vector): Check whether next
glyph code is valid before accessing it.