eval_intern.h: suppress warning by gcc 4.6

* eval_intern.h (VAR_FROM_MEMORY, VAR_INITIALIZED): gcc 4.6 also
  seems to warn false maybe-uninitialized.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46205 b2dd03c8-39d4-4d8f-98ff-823fe69b080e