tools/xenstore: simplify and rename check_event_node()

There is no path which allows to call check_event_node() without a
event name. So don't let the result depend on the name being NULL and
add an assert() covering that case.

Rename the function to check_special_event() to better match the
semantics.

This is part of XSA-115.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Paul Durrant <paul@xen.org>

tools/xenstore/xenstored_watch.c

@@ -47,13 +47,11 @@ struct watch
 	char *node;
 };
 
-static bool check_event_node(const char *node)
+static bool check_special_event(const char *name)
 {
-	if (!node || !strstarts(node, "@")) {
-		errno = EINVAL;
-		return false;
-	}
-	return true;
+	assert(name);
+
+	return strstarts(name, "@");
 }
 
 /* Is child a subnode of parent, or equal? */
@@ -87,7 +85,7 @@ static void add_event(struct connection *conn,
 	unsigned int len;
 	char *data;
 
-	if (!check_event_node(name)) {
+	if (!check_special_event(name)) {
 		/* Can this conn load node, or see that it doesn't exist? */
 		struct node *node = get_node(conn, ctx, name, XS_PERM_READ);
 		/*