tools/xenstored: fix canonicalize() error testing

The setting of errno in canonicalize() is rather fragile and seems to
be even wrong in one corner case: when the invalid path "/local/domain/"
is passed, sscanf() will set errno to 0, resulting in canonicalize() to
return NULL with errno being 0. This can result in triggering the
assert(conn->in == NULL) in consider_message().

Don't assume the initial setting of errno to "EINVAL" will stay valid
in all cases and set it to EINVAL only when returning NULL due to an
invalid path.

This is XSA-481/CVE-2026-23555

Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <julien@xen.org>
(cherry picked from commit 4c61822b04)

tools/xenstored/core.c

@@ -1242,11 +1242,10 @@ const char *canonicalize(struct connection *conn, const void *ctx,
 	 * - illegal character in node
 	 * - starts with '@' but no special node allowed
 	 */
-	errno = EINVAL;
 	if (!node ||
 	    !valid_chars(node) ||
 	    (node[0] == '@' && !allow_special))
-		return NULL;
+		goto inval;
 
 	if (node[0] != '/' && node[0] != '@') {
 		name = talloc_asprintf(ctx, "%s/%s", get_implicit_path(conn),
@@ -1274,6 +1273,8 @@ const char *canonicalize(struct connection *conn, const void *ctx,
 	if (name != node)
 		talloc_free(name);
 
+ inval:
+	errno = EINVAL;
 	return NULL;
 }