USB: serial: keyspan: fix NULL-derefs on open() and write()

Fix NULL-pointer dereferences on open() and write() which can be
triggered by a malicious USB device.

The current URB allocation helper would fail to initialise the newly
allocated URB if the device has unexpected endpoint descriptors,
something which could lead NULL-pointer dereferences in a number of
open() and write() paths when accessing the URB. For example:

	BUG: kernel NULL pointer dereference, address: 0000000000000000
	...
	RIP: 0010:usb_clear_halt+0x11/0xc0
	...
	Call Trace:
	 ? tty_port_open+0x4d/0xd0
	 keyspan_open+0x70/0x160 [keyspan]
	 serial_port_activate+0x5b/0x80 [usbserial]
	 tty_port_open+0x7b/0xd0
	 ? check_tty_count+0x43/0xa0
	 tty_open+0xf1/0x490

	BUG: kernel NULL pointer dereference, address: 0000000000000000
	...
	RIP: 0010:keyspan_write+0x14e/0x1f3 [keyspan]
	...
	Call Trace:
	 serial_write+0x43/0xa0 [usbserial]
	 n_tty_write+0x1af/0x4f0
	 ? do_wait_intr_irq+0x80/0x80
	 ? process_echoes+0x60/0x60
	 tty_write+0x13f/0x2f0

	BUG: kernel NULL pointer dereference, address: 0000000000000000
	...
	RIP: 0010:keyspan_usa26_send_setup+0x298/0x305 [keyspan]
	...
	Call Trace:
	 keyspan_open+0x10f/0x160 [keyspan]
	 serial_port_activate+0x5b/0x80 [usbserial]
	 tty_port_open+0x7b/0xd0
	 ? check_tty_count+0x43/0xa0
	 tty_open+0xf1/0x490

Fixes: fdcba53e ("fix for bugzilla #7544 (keyspan USB-to-serial converter)")
Cc: stable <stable@vger.kernel.org>	# 2.6.21
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/keyspan.c

@@ -1741,8 +1741,8 @@ static struct urb *keyspan_setup_urb(struct usb_serial *serial, int endpoint,
 
 	ep_desc = find_ep(serial, endpoint);
 	if (!ep_desc) {
-		/* leak the urb, something's wrong and the callers don't care */
-		return urb;
+		usb_free_urb(urb);
+		return NULL;
 	}
 	if (usb_endpoint_xfer_int(ep_desc)) {
 		ep_type_name = "INT";