bug symfony#9949 [BrowserKit] Throw exception on invalid cookie expiration timestamp (anlutro)

This PR was squashed before being merged into the 2.3 branch (closes symfony#9949).

Discussion
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[BrowserKit] Throw exception on invalid cookie expiration timestamp

Currently, if an invalid timestamp is provided, a fatal error with no stack trace will occur, making it difficult to trace the problem.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Commits
-------

8df535d [BrowserKit] Throw exception on invalid cookie expiration timestamp

Author: fabpot

src/Symfony/Component/BrowserKit/Cookie.php

@@ -81,14 +81,22 @@ public function __construct($name, $value, $expires = null, $path = null, $domai
      *
      * @return string The HTTP representation of the Cookie
      *
+     * @throws \UnexpectedValueException
+     *
      * @api
      */
     public function __toString()
     {
         $cookie = sprintf('%s=%s', $this->name, $this->rawValue);
 
         if (null !== $this->expires) {
-            $cookie .= '; expires='.substr(\DateTime::createFromFormat('U', $this->expires, new \DateTimeZone('GMT'))->format(self::$dateFormats[0]), 0, -5);
+            $dateTime = \DateTime::createFromFormat('U', $this->expires, new \DateTimeZone('GMT'));
+
+            if ($dateTime === false) {
+                throw new \UnexpectedValueException(sprintf('The cookie expiration time "%s" is not valid.'), $this->expires);
+            }
+
+            $cookie .= '; expires='.substr($dateTime->format(self::$dateFormats[0]), 0, -5);
         }
 
         if ('' !== $this->domain) {