Fix crash in QEMU auto-destroy with transient guests

When the auto-destroy callback runs it is supposed to return
NULL if the virDomainObjPtr is no longer valid. It was not
doing this for transient guests, so we tried to virObjectUnlock
a mutex which had been freed. This often led to a crash.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/qemu/qemu_process.c

@@ -4629,8 +4629,10 @@ qemuProcessAutoDestroy(virQEMUDriverPtr driver,
 
     if (!qemuDomainObjEndJob(driver, dom))
         dom = NULL;
-    if (dom && !dom->persistent)
+    if (dom && !dom->persistent) {
         qemuDomainRemoveInactive(driver, dom);
+        dom = NULL;
+    }
     if (event)
         qemuDomainEventQueue(driver, event);