Fix LXC driver crash when kernel doesn't support clone

* src/domain_conf.c: Make virDomainObjListFree a no-op if list
  is NULL
* src/domain_event.c: make virDomainEventCallbackListFree a no-op
  if event list is NULL
* src/lxc_driver.c: Log a message if LXC driver does not startup
  due to lacking kernel support

src/domain_conf.c

@@ -532,6 +532,9 @@ void virDomainObjListFree(virDomainObjListPtr vms)
 {
     unsigned int i;
 
+    if (!vms)
+        return;
+
     for (i = 0 ; i < vms->count ; i++)
         virDomainObjFree(vms->objs[i]);
 

src/domain_event.c

@@ -38,6 +38,9 @@ void
 virDomainEventCallbackListFree(virDomainEventCallbackListPtr list)
 {
     int i;
+    if (!list)
+        return;
+
     for (i=0; i<list->count; i++) {
         virFreeCallback freecb = list->callbacks[i]->freecb;
         if (freecb)

src/lxc_driver.c

@@ -1439,10 +1439,12 @@ static int lxcStartup(int privileged)
     lxcDriverLock(lxc_driver);
 
     /* Check that this is a container enabled kernel */
-    if(lxcContainerAvailable(0) < 0)
+    if (lxcContainerAvailable(0) < 0) {
+        VIR_INFO0("LXC support not available in this kernel, disabling driver");
         goto cleanup;
+    }
 
-    if(VIR_ALLOC(lxc_driver->domainEventCallbacks) < 0)
+    if (VIR_ALLOC(lxc_driver->domainEventCallbacks) < 0)
         goto cleanup;
     if (!(lxc_driver->domainEventQueue = virDomainEventQueueNew()))
         goto cleanup;