Fix display of truncated R2L lines on TTY frames

Fix display of truncated R2L lines on TTY frames

* src/xdisp.c (extend_face_to_end_of_line): Use a while-loop, not
a do-while loop, to avoid appending an extra glyph at the end of a
line that is one character shorter than the window-width.  This is
needed to fix display of reversed glyph rows that are almost as
wide as the window, because append_space_for_newline already added
one space glyph.