lxc: Resolve Coverity warning

Commit 'c8695053' resulted in the following:

Coverity error seen in the output:
    ERROR: REVERSE_INULL
    FUNCTION: lxcProcessAutoDestroy

Due to the 'dom' being checked before 'dom->persistent' since 'dom'
is already dereferenced prior to that.

src/lxc/lxc_process.c

@@ -75,7 +75,7 @@ lxcProcessAutoDestroy(virDomainObjPtr dom,
                                      VIR_DOMAIN_EVENT_STOPPED_DESTROYED);
     priv->doneStopEvent = true;
 
-    if (dom && !dom->persistent) {
+    if (!dom->persistent) {
         virDomainObjListRemove(driver->domains, dom);
         dom = NULL;
     }