bind_at_call

# File ext/dl/lib/dl/func.rb, line 236
    def bind_at_call(&block)
      userdata = @carrier[-1]
      userdata[0].push(block)
      bind{|*args|
        ptr = args[@index]
        if( !ptr )
          raise(RuntimeError, "The index of userdata should be lower than #{args.size}.")
        end
        userdata = dlunwrap(Integer(ptr))
        args[@index] = userdata[1]
        userdata[0][0].call(*args)
      }
      @mutex.unlock()
    end