Really don't crash if a connection closes early

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>
(cherry picked from commit 066c8ef6)

src/rpc/virnetserverclient.c

@@ -1540,7 +1540,7 @@ virNetServerClientStartKeepAlive(virNetServerClientPtr client)
     /* The connection might have been closed before we got here and thus the
      * keepalive object could have been removed too.
      */
-    if (!client->sock) {
+    if (!client->keepalive) {
         virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                        _("connection not open"));
         goto cleanup;