Temporarily suspend PHP Warnings on invalid tags

Temporarily suspend PHP Warnings on invalid tags when processing Gist
HTML by setting the libxml_use_internal_errors() value to true.

The original value for libxml_use_internal_errors() is restored when the
processing is complete.

props @Dan0sz

See #80

Resolves #81

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## [Unreleased]
 
+* Temporarily suspend PHP Warnings on invalid tags when processing Gist HTML ([#81](https://github.com/bradyvercher/gistpress/issues/81))
+
 ## [v3.0.2] - 2020-01-16
 
 * Sanitized the `id` attribute passed to the `[gist]` shortcode. This fixes an XSS vulnerability that could be exploited by untrusted contributors on multi-author sites. Thanks to [@cornerpirate](https://github.com/cornerpirate) for disclosing responsibly.

includes/class-gistpress.php

@@ -463,6 +463,10 @@ public function process_gist_html( $html, array $args ) {
 		$html = '<?xml encoding="utf-8" ?>' . $html;
 
 		$dom = new DOMDocument();
+
+		// Temporarily suppress warnings for invalid tags.
+		$previous_libxml_use_internal_errors_value = libxml_use_internal_errors( true );
+
 		$dom->loadHTML( $html, LIBXML_HTML_NODEFDTD | LIBXML_HTML_NOIMPLIED );
 
 		$lines = $dom->getElementsByTagName( 'tr' );
@@ -537,6 +541,9 @@ public function process_gist_html( $html, array $args ) {
 			$html = $this->process_gist_line_numbers( $html, $args['lines'], $args['lines_start'] );
 		}
 
+		// Reset to previous value.
+		libxml_use_internal_errors( $previous_libxml_use_internal_errors_value );
+
 		return $html;
 	}