Add boolean immutable flag to the DOM stack state

Can be used to ensure the document root node never leaves the stack while
parsing.

src/document/dom/stack.c

@@ -140,6 +140,9 @@ do_pop_dom_node(struct dom_stack *stack, struct dom_stack_state *parent)
 	assert(stack && !dom_stack_is_empty(stack));
 
 	state = get_dom_stack_top(stack);
+	if (state->immutable)
+		return 1;
+
 	callback = stack->pop_callbacks[state->node->type];
 	if (callback) {
 		void *state_data = get_dom_stack_state_data(stack, state);

src/document/dom/stack.h

@@ -27,6 +27,9 @@ struct dom_stack_state {
 	/* The depth of the state in the stack. This is amongst other things
 	 * used to get the state object data. */
 	unsigned int depth;
+
+	/* Wether this stack state can be popped with pop_dom_*() family. */
+	unsigned int immutable:1;
 };
 
 /* The DOM stack is a convenient way to traverse DOM trees. Also it

src/document/sgml/parser.c

@@ -330,6 +330,8 @@ init_sgml_parser(enum sgml_parser_type type, void *data, struct uri *uri,
 		return NULL;
 	}
 
+	get_dom_stack_top(&parser->stack)->immutable = 1;
+
 	return parser;
 }