HTML API: Fix a fatal error when processing malformed document with unclosed attribute.

Props: dlh, costdev, dmsnell.
Fixes: #58637.

git-svn-id: https://develop.svn.wordpress.org/trunk@56133 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -546,6 +546,10 @@ class WP_HTML_Tag_Processor {
 			}
 
 			// Ensure that the tag closes before the end of the document.
+			if ( $this->bytes_already_parsed >= strlen( $this->html ) ) {
+				return false;
+			}
+
 			$tag_ends_at = strpos( $this->html, '>', $this->bytes_already_parsed );
 			if ( false === $tag_ends_at ) {
 				return false;