Fix 'vertical-motion' and 'posn-at-point' under 'visual-line-mode'

Fix 'vertical-motion' and 'posn-at-point' under 'visual-line-mode'

* src/xdisp.c (move_it_in_display_line_to): Don't assume we can
wrap on a whitespace character if it's followed by another
whitespace character.  When returning under WORD_WRAP for a screen
line that is continued, restore to wrap point when atpos/atx
position would be displayed on the next screen line due to
line-wrap.  (Bug#23570)

This is backported from master
(cherry picked from commit 99848b37d2c3e14c0af45fc6da437a806aa58a80)