array.c: swap volatile for RB_GC_GUARD

* array.c (ary_recycle_hash): add RB_GC_GUARD
  (rb_ary_diff): remove volatile
  [Bug #10369]

The RB_GC_GUARD seems needed for most of the ary_recycle_hash
functions anyways.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47896 b2dd03c8-39d4-4d8f-98ff-823fe69b080e