minor #5128 [VarDumper] little optim (lyrixx)

This PR was submitted for the 2.7 branch but it was merged into the 2.6 branch instead (closes #5128).

Discussion
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[VarDumper] little optim

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | 2.7+
| Fixed tickets | -

related to symfony/symfony#14137
and asked by @nicolas-grekas

Commits
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a880fe4 [VarDumper] little optim

components/var_dumper/advanced.rst

@@ -145,8 +145,7 @@ Another option for doing the same could be::
     $output = fopen('php://memory', 'r+b');
 
     $dumper->dump($cloner->cloneVar($variable), $output);
-    rewind($output);
-    $output = stream_get_contents($output);
+    $output = stream_get_contents($output, -1, 0);
 
     // $output is now populated with the dump representation of $variable