string.c: fix false coderange

	* string.c (rb_enc_str_scrub): enc can differ from the actual
	  encoding of the string, the cached coderange is useless then.
	  [ruby-core:82674] [Bug #13874]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@59822 b2dd03c8-39d4-4d8f-98ff-823fe69b080e