tq: verify connection and return error on nil

In some cases, we can end up with a panic due to a pure SSH transfer
lacking any connections, but it's not clear what those cases are.  Let's
add some explicit checks that our connection is functional and return an
error if not.

Author: bk2204

tq/ssh.go

@@ -27,6 +27,9 @@ type SSHBatchClient struct {
 
 func (a *SSHBatchClient) batchInternal(args []string, batchLines []string) (int, []string, []string, error) {
 	conn := a.transfer.Connection(0)
+	if conn == nil {
+		return 0, nil, nil, errors.Errorf(tr.Tr.Get("could not get connection for batch request"))
+	}
 	conn.Lock()
 	defer conn.Unlock()
 	err := conn.SendMessageWithLines("batch", args, batchLines)
@@ -179,6 +182,9 @@ func (a *SSHAdapter) DoTransfer(ctx interface{}, t *Transfer, cb ProgressCallbac
 		authOkFunc()
 	}
 	conn := ctx.(*ssh.PktlineConnection)
+	if conn == nil {
+		return errors.Errorf(tr.Tr.Get("could not get connection for transfer"))
+	}
 	if a.adapterBase.direction == Upload {
 		return a.upload(t, conn, cb)
 	} else {