Fix disappearance of command_call in case of statement without parentheses

This patch fixes following bug.

    def foo(a, b)
      return a, b
    end

    a, b = foo 1,2
    a, b = foo(1,2)
    p a, b

before the fix:
    Object
    nil
    1
    main

after the fix:
    1
    2
    1
    2