Fix memory leak

	* string.c (str_make_independent_expand): free independent buffer.
	  [Bug# 15935]

Co-Authored-By: luke-gru (Luke Gruber) <luke.gru@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67805 b2dd03c8-39d4-4d8f-98ff-823fe69b080e