array.c: comment why rb_ary_modify is needed twice

* array.c (rb_ary_initialize): NUM2LONG() may call size.to_int,
  ary can be frozen, modified, etc, so recheck after argument
  conversion is necessary.  [Closes GH-526]

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44855 b2dd03c8-39d4-4d8f-98ff-823fe69b080e