string.c: fix capacity

* string.c (str_buf_cat): should round up the capacity by 4KiB,
  but not number of rooms.  [ruby-core:61886] [Bug #9709]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45534 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-04-09

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45535 b2dd03c8-39d4-4d8f-98ff-823fe69b080e