array.c: fix potential memory leak

* array.c (ary_new): allocate buffer in heap after new object get
  allocated successfully, to get rid of potential memory leak at
  object allocation failure.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47855 b2dd03c8-39d4-4d8f-98ff-823fe69b080e