The fixedbugs/issue12536.go file was erroneously deleted just before
committing the patch that fixed the issue (CL 14400).

That's an easy test and there's a small reproducer in the issue, add
it back.

Updates #12536

Change-Id: Ib7b0cd245588299e9a5469e1d75805fd0261ce1a
Reviewed-on: https://go-review.googlesource.com/40712Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>