host/logbuf: Fix goroutine leak

Before this change, we’d keep hitting the l.closed path through the
select and keep creating new goroutines. Instead, use a copy of the
channel and set it to nil after triggering the StopAtEOF goroutine,
which ensures the path won’t be hit again.

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Merge pull request #678 from flynn/fix-logbuf-leak

host/logbuf: Fix goroutine leak