xenstore: handle do_mkdir and do_rm failure

xenstore: handle do_mkdir and do_rm failure

In the out of memory case, we might return a NULL pointer when
canonicalizing node names. This NULL pointer is not checked when
creating a directory, or when removing a node. This change handles
the NULL pointer for these two cases.

This bug was discovered and resolved using Coverity Static Analysis
Security Testing (SAST) by Synopsys, Inc.

Signed-off-by: Norbert Manthey <nmanthey@amazon.de>
Reviewed-by: Thomas Friebel <friebelt@amazon.de>
Reviewed-by: Julien Grall <jgrall@amazon.co.uk>
Reviewed-by: Juergen Gross <jgross@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>