* misc/ruby-mode.el: Use preceding-char/following-char
  (returning 0 at BOF/EOF) instead of char-before/char-after
  (returning nil at BOF/EOF) to avoid error from char-syntax when
  at BOF/EOF.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43189 b2dd03c8-39d4-4d8f-98ff-823fe69b080e