Ensure that we call FinishType on all the types added to the ptrs map.
We only add a key to ptrKeys once. Once we FinishType for that key,
we'll never look at that key again. But we can add a new type under that
key later, and we'll never finish it.

Make sure we add the key to the ptrKeys list every time we make the list
of types for that key non-empty.

This makes sure we FinishType each pointer type exactly once.

Fixes #26517

Change-Id: Iad86150d516fcfac167591daf5a26c38bec7d143
Reviewed-on: https://go-review.googlesource.com/126275Reviewed-by: Ian Lance Taylor <iant@golang.org>