Avoid crashes when a reversed glyph row starts with a composition

Avoid crashes when a reversed glyph row starts with a composition

* src/dispnew.c (build_frame_matrix_from_leaf_window): Add an
assertion to prevent us from overwriting non-char glyphs with the
vertical border glyph.
* src/xdisp.c (extend_face_to_end_of_line): Account for one glyph
possibly inserted by append_space_for_newline.  (Bug#44506)
Remove a kludgey correction for an off-by-one error in column
counting, which is no longer needed.