lxc: Resolve Coverity RESOURCE_LEAK

Commit id 'c27553b6' added a return -1 in a failure path without
the necessary VIR_FREE(stack)

src/lxc/lxc_container.c

@@ -2463,6 +2463,7 @@ int lxcContainerStart(virDomainDefPtr def,
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                            _("Config askes for inherit net namespace "
                              "as well as private network interfaces"));
+            VIR_FREE(stack);
             return -1;
         }
         VIR_DEBUG("Inheriting a net namespace");