Catch another indexing operation

Author: dmsnell

src/wp-includes/html-api/class-wp-html-tag-processor.php

@@ -1036,7 +1036,7 @@ private function parse_next_tag() {
 				 * https://html.spec.whatwg.org/multipage/parsing.html#tag-open-state
 				 */
 				if (
-					strlen( $html ) > $at + 3 &&
+					strlen( $html ) > $at + 4 &&
 					'-' === $html[ $at + 2 ] &&
 					'-' === $html[ $at + 3 ]
 				) {

tests/phpunit/tests/html-api/wpHtmlTagProcessor.php

@@ -1800,6 +1800,7 @@ public function test_documents_may_end_with_unclosed_comment( $html_ending_befor
 	 */
 	public function data_html_with_unclosed_comments() {
 		return array(
+			'Shortest open valid comment'      => array( '<!--' ),
 			'Basic truncated comment'          => array( '<!-- this ends --' ),
 			'Comment with closer look-alike'   => array( '<!-- this ends --x' ),
 			'Comment with closer look-alike 2' => array( '<!-- this ends --!x' ),