bug #19379 [VarDumper] Fix for PHP 7.1 (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
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[VarDumper] Fix for PHP 7.1

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #19377
| License       | MIT
| Doc PR        | -

Tests fail on 7.1 otherwise

Commits
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d422939 [VarDumper] Fix for 7.1

src/Symfony/Component/VarDumper/Cloner/VarCloner.php

@@ -315,7 +315,7 @@ class VarCloner extends AbstractCloner
             if (!empty($frame['line'])) {
                 ob_start();
                 debug_zval_dump($obj);
-                self::$hashMask = substr(ob_get_clean(), 17);
+                self::$hashMask = (int) substr(ob_get_clean(), 17);
             }
         }