rpc: Don't unref identity object while callbacks still can be executed

While this thread is cleaning up the client and connection objects:
 #2  virFileReadAll (path=0x7f28780012b0 "/proc/1319/stat", maxlen=maxlen@entry=1024, buf=buf@entry=0x7f289c60fc40) at util/virfile.c:1287
 #3  0x00007f28adbb1539 in virProcessGetStartTime (pid=<optimized out>, timestamp=timestamp@entry=0x7f289c60fc98) at util/virprocess.c:838
 #4  0x00007f28adb91981 in virIdentityGetSystem () at util/viridentity.c:151
 #5  0x00007f28ae73f17c in remoteClientFreeFunc (data=<optimized out>) at remote.c:1131
 #6  0x00007f28adcb7f33 in virNetServerClientDispose (obj=0x7f28aecad180) at rpc/virnetserverclient.c:858
 #7  0x00007f28adba8eeb in virObjectUnref (anyobj=<optimized out>) at util/virobject.c:265
 #8  0x00007f28ae74ad05 in virNetServerHandleJob (jobOpaque=<optimized out>, opaque=0x7f28aec93ff0) at rpc/virnetserver.c:205
 #9  0x00007f28adbbef4e in virThreadPoolWorker (opaque=opaque@entry=0x7f28aec88030) at util/virthreadpool.c:145

In stack frame #6 the client->identity object got unref'd, but the code
that removes the event callbacks in frame #5 did not run yet as we are
trying to obtain the system identity (frames #4, #3, #2).

In other thead:
 #0  virObjectUnref (anyobj=anyobj@entry=0x7f288c162c60) at util/virobject.c:264
        klass = 0xdeadbeef
        obj = 0x7f288c162c60
 #1  0x00007f28ae71c709 in remoteRelayDomainEventCheckACL (client=<optimized out>, conn=<optimized out>, dom=dom@entry=0x7f28aecaafc0) at remote.c:164
 #2  0x00007f28ae71fc83 in remoteRelayDomainEventTrayChange (conn=<optimized out>, dom=0x7f28aecaafc0, ... ) at remote.c:717
 #3  0x00007f28adc04e53 in virDomainEventDispatchDefaultFunc (conn=0x7f287c0009a0, event=0x7f28aecab1a0, ...) at conf/domain_event.c:1455
 #4  0x00007f28adc03831 in virObjectEventStateDispatchCallbacks (callbacks=<optimized out>, ....) at conf/object_event.c:724
 #5  virObjectEventStateQueueDispatch (callbacks=0x7f288c083730, queue=0x7fff51f90030, state=0x7f288c18da20) at conf/object_event.c:738
 #6  virObjectEventStateFlush (state=0x7f288c18da20) at conf/object_event.c:816
 #7  virObjectEventTimer (timer=<optimized out>, opaque=0x7f288c18da20) at conf/object_event.c:562
 #8  0x00007f28adb859cd in virEventPollDispatchTimeouts () at util/vireventpoll.c:459

Frame #0 is unrefing an invalid identity object while frame #2 hints
that the client is still dispatching the event.

For untrimmed backtrace see the bugzilla attachment.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1203030

src/rpc/virnetserverclient.c

@@ -850,12 +850,12 @@ void virNetServerClientDispose(void *obj)
     PROBE(RPC_SERVER_CLIENT_DISPOSE,
           "client=%p", client);
 
-    virObjectUnref(client->identity);
-
     if (client->privateData &&
         client->privateDataFreeFunc)
         client->privateDataFreeFunc(client->privateData);
 
+    virObjectUnref(client->identity);
+
 #if WITH_SASL
     virObjectUnref(client->sasl);
 #endif