process.c: variable as macro argument

* process.c (run_exec_open): use a local variable to get rid of
  RARRAY_AREF() is evaluated multiple times by RSTRING_PTR().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46439 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-06-16

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46440 b2dd03c8-39d4-4d8f-98ff-823fe69b080e