https://bugzilla.redhat.com/show_bug.cgi?id=1047577

When writing commit 173c2914, I missed the fact virNetServerClientClose
unlocks the client object before actually clearing client->sock and thus
it is possible to hit a window when client->keepalive is NULL while
client->sock is not NULL. I was thinking client->sock == NULL was a
better check for a closed connection but apparently we have to go with
client->keepalive == NULL to actually fix the crash.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>