Commit of #18dd60c1 causes disappearance of a local variable
if return statement is called without arguments.

This patch fixes incorrect value of stack pointer.

test program:
def test_return_cond
  return if nil
  obj = 123
  p obj
  p obj.class
end
test_return_cond

output(commit #18dd60c1):
main
Object

output(includes this patch):
123
Fixnum