Read what a non-empty interface points to.

The deleted lines were added in https://codereview.appspot.com/4810060/,
which attempted to break an infinite loop. That was a long time ago.
If I just delete these lines with current codebase, the test "bug1"
(added in that CL) does not fail.

All new tests fail without this fix.

Fixes #12924

Change-Id: I9370ca44facd6af3019850aa065b936e5a482d37
Reviewed-on: https://go-review.googlesource.com/15809Reviewed-by: Andrew Gerrand <adg@golang.org>