Parser: Handle regex failure in extractBody method

Add a check for regex failure in the extractBody method and throw
a RuntimeException with the error details if preg_replace returns null.

Bug: T388729
Change-Id: I59abad3a58ccd6edc6517b13a56d8253ba0e0928
(cherry picked from commit 3b297d37dd368d1d66f7afd78851bbb7a47cab0b)

includes/parser/Parser.php

@@ -6484,6 +6484,10 @@ class Parser {
 	 */
 	public static function extractBody( string $text ): string {
 		$text = preg_replace( '!^.*?<body[^>]*>!s', '', $text, 1 );
+		if ( $text === null ) {
+			// T388729: this should never happen
+			throw new RuntimeException( 'Regex failed: ' . preg_last_error() );
+		}
 		$text = preg_replace( '!</body>\s*</html>\s*$!', '', $text, 1 );
 		return $text;
 	}