access: report an error if no access manager is present

The code calling this method expects it to have reported an error on
failure.
Reviewed-by: NCole Robinson <crobinso@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/access/viraccessmanager.c

@@ -65,6 +65,12 @@ VIR_ONCE_GLOBAL_INIT(virAccessManager);
 
 virAccessManagerPtr virAccessManagerGetDefault(void)
 {
+    if (virAccessManagerDefault == NULL) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("No access manager registered"));
+        return NULL;
+    }
+
     return virObjectRef(virAccessManagerDefault);
 }