Restore previous state for libxml option

Whenever libxml_use_internal_errors() is called, the previous value for
it should be restored. This used to be the case in this piece of code,
but it was wrongly removed in e53bf5839b ,
which has the nasty side effect of making the Validator component test
suite break with this message:

Validation failed: No DTD found!

src/Symfony/Component/Translation/Command/XliffLintCommand.php

@@ -115,7 +115,7 @@ EOF
             return ['file' => $file, 'valid' => true];
         }
 
-        libxml_use_internal_errors(true);
+        $internal = libxml_use_internal_errors(true);
 
         $document = new \DOMDocument();
         $document->loadXML($content);
@@ -143,6 +143,9 @@ EOF
             ];
         }
 
+        libxml_clear_errors();
+        libxml_use_internal_errors($internal);
+
         return ['file' => $file, 'valid' => 0 === \count($errors), 'messages' => $errors];
     }