cmd/buildlet: cleanup dead coordinator net.Conn

The farmer.golang.org flapping got the reverse buildlets stuck.

Tested manually.

Change-Id: I0e7dca2747b2c36bb8c39c41ebaaf4bf997c6f35
Reviewed-on: https://go-review.googlesource.com/9762
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed