Fix uninitialized variable in virLXCControllerSetupDevPTS

The lack of initialization of 'opts' caused a SEGV in the
cleanup: path if the root->src directory did not exist

src/lxc/lxc_controller.c

@@ -1144,7 +1144,7 @@ virLXCControllerSetupDevPTS(virLXCControllerPtr ctrl)
 {
     virDomainFSDefPtr root = virDomainGetRootFilesystem(ctrl->def);
     char *mount_options = NULL;
-    char *opts;
+    char *opts = NULL;
     char *devpts = NULL;
     int ret = -1;