compile.c: defined? with empty expression

* compile.c (iseq_compile_each): fix of defined? with empty
  expression.  [ruby-core:53999] [Bug #8220]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40124 b2dd03c8-39d4-4d8f-98ff-823fe69b080e