guile: fix bad conversion of shared strings (replace calls to scm_i_string_chars by scm_to_locale_string) (bug #38067)
core: fix crash in display of chat area when the ncurses chat window is not initialized

This can happen when a window becomes too small for display: then we set the
ncurses chat window to NULL.
So when displaying buffer, we'll skip it if the ncurses chat window is NULL.

Steps to reproduce crash:
  1. /window splith
  2. /window resize -1 (many times, until chat area is less than one line)
  3. /window balance
doc: update polish FAQ, quickstart and scripting guide