kern_jail_set(): do not double-free opts

If vfs_buildopts() failed, vfs_freeopts() must not be called.

Reported and tested by:	pho
Reviewed by:	jamie
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Differential revision:	https://reviews.freebsd.org/D53612

sys/kern/kern_jail.c

@@ -1065,8 +1065,10 @@ kern_jail_set(struct thread *td, struct uio *optuio, int flags)
 	 *     than duplicate it under a different name.
 	 */
 	error = vfs_buildopts(optuio, &opts);
-	if (error)
+	if (error) {
+		opts = NULL;
 		goto done_free;
+	}
 
 	cuflags = flags & (JAIL_CREATE | JAIL_UPDATE);
 	if (!cuflags) {
@@ -2331,7 +2333,8 @@ kern_jail_set(struct thread *td, struct uio *optuio, int flags)
 		(void)kern_close(td, jfd_out);
 	if (g_path != NULL)
 		free(g_path, M_TEMP);
-	vfs_freeopts(opts);
+	if (opts != NULL)
+		vfs_freeopts(opts);
 	prison_free(mypr);
 	return (error);
 }