string.c: fix scanned substring with `\K`

	* string.c (scan_once): fix the matched substring with `\K`, the
	  beginning of that string may differ from the matched position.
	  [ruby-core:86663] [Bug #14707]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@64320 b2dd03c8-39d4-4d8f-98ff-823fe69b080e