string.c: remove unnecessary terminator space

* string.c (str_buf_cat): remove unnecessary terminator space,
  since the capacity does not include its length but RESIZE_CAPA()
  considers it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45536 b2dd03c8-39d4-4d8f-98ff-823fe69b080e