Fixes #16570 on iOS.

Thanks Daniel Burhans for reporting the bug and testing the fix.

Change-Id: I43ae7b78c8f85a131ed3d93ea59da9f32a02cd8f
Reviewed-on: https://go-review.googlesource.com/25481Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>