lxc: Avoid segfault of libvirt_lxc helper on early cleanup paths

Early jumps to the cleanup label caused a crash of the libvirt_lxc
container helper as the cleanup section called
virLXCControllerDeleteInterfaces(ctrl) without checking the ctrl argument
for NULL. The argument was de-referenced soon after.

$ /usr/libexec/libvirt_lxc
/usr/libexec/libvirt_lxc: missing --name argument for configuration
Segmentation fault

src/lxc/lxc_controller.c

@@ -1672,7 +1672,8 @@ int main(int argc, char *argv[])
 
 cleanup:
     virPidFileDelete(LXC_STATE_DIR, name);
-    virLXCControllerDeleteInterfaces(ctrl);
+    if (ctrl)
+        virLXCControllerDeleteInterfaces(ctrl);
     for (i = 0 ; i < nttyFDs ; i++)
         VIR_FORCE_CLOSE(ttyFDs[i]);
     VIR_FREE(ttyFDs);