string.c: use local variables

* string.c (rb_str_succ): extract local variables from constant
  RSTRING_PTR and RSTRING_LEN.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48780 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
string.c: term fill

* string.c (rb_str_succ): fill wchar terminator.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48781 b2dd03c8-39d4-4d8f-98ff-823fe69b080e