string.c: clear substring code range

	* string.c (str_substr): substring of broken code range string may
	  be valid or broken.  patch by tommy (Masahiro Tomita) at
	  [ruby-dev:50430] [Bug #14388].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@62946 b2dd03c8-39d4-4d8f-98ff-823fe69b080e