Early simple return of connectivity check. Set connection to null on disconnect.

Author: frankdejonge

src/PhpseclibV3/SftpConnectionProvider.php

@@ -87,6 +87,7 @@ public function disconnect(): void
     {
         if ($this->connection) {
             $this->connection->disconnect();
+            $this->connection = null;
         }
     }
 

src/PhpseclibV3/SimpleConnectivityChecker.php

@@ -11,6 +11,10 @@ class SimpleConnectivityChecker implements ConnectivityChecker
 {
     public function isConnected(SFTP $connection): bool
     {
+        if ( ! $connection->isConnected()) {
+            return false;
+        }
+
         try {
             return $connection->ping();
         } catch (Throwable) {