Ensure diagnose command works even if provider-includes disappears

src/Composer/Command/DiagnoseCommand.php

@@ -328,6 +328,7 @@ EOT
             }
 
             $json = $this->httpDownloader->get($protocol.'://repo.packagist.org/packages.json')->decodeJson();
+            if (isset($json['provider-includes'])) {
                 $hash = reset($json['provider-includes']);
                 $hash = $hash['sha256'];
                 $path = str_replace('%hash%', $hash, key($json['provider-includes']));
@@ -336,6 +337,7 @@ EOT
                 if (hash('sha256', $provider) !== $hash) {
                     return '<warning>It seems that your proxy ('.$proxyStatus.') is modifying '.$protocol.' traffic on the fly</>';
                 }
+            }
 
             return '<info>OK</> <comment>'.$proxyStatus.'</>';
         } catch (\Exception $e) {