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>