The default implementation of Accept, which spins up a new server
for every new connection, calls log.Fatal if the listener is closed,
stopping any outstanding work. Change that to a non-fatal log
call so work can continue.

There is no programmatic signaling of the problem, just the log,
but that should be enough.

Fixes #11221.

Change-Id: I7c7f6164a0a0143236729eb778d7638c51c34ed1
Reviewed-on: https://go-review.googlesource.com/14185Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>