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.
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