Fix 'end-of-visual-line' with overlay strings with newlines

Fix 'end-of-visual-line' with overlay strings with newlines

* src/indent.c (Fvertical_motion): Get out of overlay strings
with embedded newlines even if moving within the same screen
line.  See https://github.com/emacs-lsp/lsp-mode/issues/677
for more details.
Fix test for IT_CHARPOS being at the beginning of the
accessible portion of the buffer.