merge revision(s) 39495: [Backport #7954]

	* string.c (str_byte_substr): don't set coderange if it's not known.
	  [Bug #7954] [ruby-dev:47108]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39706 b2dd03c8-39d4-4d8f-98ff-823fe69b080e