fmt: fix internal unknownType function
This thing should never be called, but before
151960044 it was being called, incorrectly.
This is now just a precaution but let's pretend it
Fixes issue 8843
even though that was fixed by 151960044.
The test case was already there and ran, another mystery.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/151970043