In "if (saddr->keep_alive) {" we may already be on error path, with
invalid sock < 0. Fix it by returning error earlier.

Reported-by: Coverity (CID 1405300)
Fixes: aec21d31
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190910075943.12977-1-vsementsov@virtuozzo.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>