io.c: remove redundant assignment

* io.c (rb_io_sysread): Remove redundan assignment of 'n'.
  [Fix GH-767]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48538 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-11-23

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48539 b2dd03c8-39d4-4d8f-98ff-823fe69b080e