* Avoid undefined behaviors found by gcc -fsanitize=undefined.
  gcc (Debian 4.9.1-16) 4.9.1

* string.c (rb_str_sum): Avoid undefined behavior.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47997 b2dd03c8-39d4-4d8f-98ff-823fe69b080e