Verify that Qnil is zero before relying on that in redisplay.

Verify that Qnil is zero before relying on that in redisplay.

 src/dispnew.c (adjust_glyph_matrix, realloc_glyph_pool): Verify that
 Qnil is represented as zero, before using that to initialize parts
 of the glyph structure.
 src/xdisp.c (init_iterator): Verify that Qnil is represented as
 zero, before using that to initialize parts of the iterator
 structure.