net/http: fix possible nil pointer dereference in TestOnlyWriteTimeout

net/http: fix possible nil pointer dereference in TestOnlyWriteTimeout

TestOnlyWriteTimeout assumes wrongly that:
- the Accept method of trackLastConnListener is called only once
- the shared variable conn never becomes nil
and crashes on some circumstances.

Updates #19032.

Change-Id: I61de22618cd90b84a2b6401afdb6e5d9b3336b12
Reviewed-on: https://go-review.googlesource.com/36735
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>