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/trunk@63252 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_string.rb

@@ -1542,6 +1542,8 @@ CODE
     assert_nil($~)
 
     assert_equal(3, S("hello hello hello").scan("hello".taint).count(&:tainted?))
+
+    assert_equal(%w[1 2 3], S("a1 a2 a3").scan(/a\K./))
   end
 
   def test_size