stringio.c: check character code

	* ext/stringio/stringio.c (strio_ungetc): check if the character
	  code is valid in the encoding.  reported by Ahmad Sherif
	  (ahmadsherif) at https://hackerone.com/reports/209593.
	stringio.c: check range

	* ext/stringio/stringio.c (strio_ungetc): raise RangeError instead
	  of TypeError at too big value, as well as IO#ungetc.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@58052 b2dd03c8-39d4-4d8f-98ff-823fe69b080e