Fix bug #17244 with line-move-visual when display string covers a lot of text.

Fix bug #17244 with line-move-visual when display string covers a lot of text.

 src/xdisp.c (move_it_by_lines): If a large portion of buffer text is
 covered by a display string that ends in a newline, and that cases
 going back by DVPOS lines to hit the search limit, lift the limit
 and go back until DVPOS is reached.
 src/indent.c (Fvertical_motion): Handle correctly the case when the
 display string is preceded by an empty line.