PHPMailer::createHeader: minor tweak to messageID validation (#2391)

Minor improvement to the *massive* improvement from 853afcb858.

The `$` in a regex means "match the end of a string or new line right before the end". Using the `D` modifier prevents the matching on the new line.

Ref:
* https://www.php.net/manual/en/reference.pcre.pattern.modifiers.php
* http://web.archive.org/web/20131226183006/http://blog.php-security.org/archives/76-Holes-in-most-preg_match-filters.html

Co-authored-by: jrfnl <jrfnl@users.noreply.github.com>

src/PHPMailer.php

@@ -2560,7 +2560,7 @@ class PHPMailer
                 '|("(([\x01-\x08\x0B\x0C\x0E-\x1F\x7F]|[\x21\x23-\x5B\x5D-\x7E])' .
                 '|(\\[\x01-\x09\x0B\x0C\x0E-\x7F]))*"))@(([a-zA-Z0-9!#$%&\'*+\/=?^_`{|}~-]+' .
                 '(\.[a-zA-Z0-9!#$%&\'*+\/=?^_`{|}~-]+)*)|(\[(([\x01-\x08\x0B\x0C\x0E-\x1F\x7F]' .
-                '|[\x21-\x5A\x5E-\x7E])|(\\[\x01-\x09\x0B\x0C\x0E-\x7F]))*\])))>$/',
+                '|[\x21-\x5A\x5E-\x7E])|(\\[\x01-\x09\x0B\x0C\x0E-\x7F]))*\])))>$/D',
                 $this->MessageID
             )
         ) {

test/PHPMailer/GetLastMessageIDTest.php

@@ -54,6 +54,7 @@ final class GetLastMessageIDTest extends TestCase
             'Invalid: plain hash' => [$hash],
             'Invalid: missing brackets' => [$hash . '@example.com'],
             'Invalid: missing @' => ['<' . $hash . 'example.com>'],
+            'Invalid: new line after bracket' => ['<' . $hash . "@example.com>\n"],
             'Invalid: no text before @' => ['<@example.com>'],
             'Invalid: no text after @' => ['<' . $hash . '@>'],
             'Invalid: no text before or after @' => ['<@>'],
@@ -91,7 +92,7 @@ final class GetLastMessageIDTest extends TestCase
         $hash = hash('sha256', 12345);
 
         return [
-            'hashed pre @'               => [ '<' . $hash . '@example.com>' ],
+            'hashed pre @' => ['<' . $hash . '@example.com>'],
         ];
     }