xenstore: add missing NULL check

In case of allocation error, we should not dereference the obtained
NULL pointer.

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

Signed-off-by: Michael Kurth <mku@amazon.com>
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>

tools/xenstore/xenstored_core.c

@@ -504,6 +504,11 @@ int write_node_raw(struct connection *conn, TDB_DATA *key, struct node *node,
 	}
 
 	data.dptr = talloc_size(node, data.dsize);
+	if (!data.dptr) {
+		errno = ENOMEM;
+		return errno;
+	}
+
 	hdr = (void *)data.dptr;
 	hdr->generation = node->generation;
 	hdr->num_perms = node->perms.num;