http2: fix channel double-close crash

When an active http2 connection closed, a stream's done channel could
be closed twice and panic.

Fixes golang/go#13924

Change-Id: I5e050b2788070321131ca2756a899f968e9bc0b2
Reviewed-on: https://go-review.googlesource.com/18571
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed