Fix error in default target proposal client

Reason is that defined? need real expression that it can evaluate
specially so it not raise exception.

Code I use to verify it:

class A
  def a
    puts "OK"
  end
end unless defined? A

class A
  def a
    puts "Fail"
  end
end unless defined? A

A.new.a
release
Merge pull request #53 from yast/fix_error

Fix error