smb: fix TCP connection leak when connection setup fails - fixes #9678

If revealing the password, creating the Kerberos client or the SMB
handshake failed after the TCP connection was established, the
connection was never closed.

backend/smb/connpool.go

@@ -21,12 +21,18 @@ import (
 // initiates the SMB handshake, and then sets up a Client.
 //
 // The context is only used for establishing the connection, not after.
-func (f *Fs) dial(ctx context.Context, network, addr string) (*conn, error) {
+func (f *Fs) dial(ctx context.Context, network, addr string) (c *conn, err error) {
 	dialer := fshttp.NewDialer(ctx)
 	tconn, err := dialer.DialContext(ctx, network, addr)
 	if err != nil {
 		return nil, err
 	}
+	// Close tconn it if any of the remaining setup fails.
+	defer func() {
+		if err != nil {
+			_ = tconn.Close()
+		}
+	}()
 
 	pass := ""
 	if f.opt.Pass != "" {