qemu: avoid null deref on low memory

Detected by Coverity.  qemuDomainEventQueue requires a non-NULL
pointer; most callers silently drop the event if we encountered
and OOM situation trying to create the event.

* src/qemu/qemu_migration.c (qemuMigrationFinish): Check for OOM.

src/qemu/qemu_migration.c

@@ -2552,7 +2552,8 @@ qemuMigrationFinish(struct qemud_driver *driver,
                                          VIR_DOMAIN_EVENT_RESUMED_MIGRATED);
         if (virDomainObjGetState(vm, NULL) == VIR_DOMAIN_PAUSED) {
             virDomainObjSetState(vm, VIR_DOMAIN_PAUSED, VIR_DOMAIN_PAUSED_USER);
-            qemuDomainEventQueue(driver, event);
+            if (event)
+                qemuDomainEventQueue(driver, event);
             event = virDomainEventNewFromObj(vm,
                                              VIR_DOMAIN_EVENT_SUSPENDED,
                                              VIR_DOMAIN_EVENT_SUSPENDED_PAUSED);