tty: serial: jsm: Add one check against NULL pointer dereference

All calls to neo_copy_data_from_uart_to_queue() are safeguarded
against NULL dereference of its parameter, except the one that
this patch changes.

That said, let's play safe and check for NULL in this case too.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>