workaround for NoMethodError when the original of alias is not found.

[nobu]

RDoc calls `documented?' on a constant name string, when another constant refers it but the file where the original is defined is skipped because it has not been modified after the previous rdoc generation.
ruby/ruby@50ffa68

[drbrain]

I committed it with @a883e14b. I will try to write a test, too