comedi: ni_usb6501: fix NULL-deref in command paths

In the Linux kernel, the following vulnerability has been resolved:

comedi: ni_usb6501: fix NULL-deref in command paths

The driver uses endpoint-sized USB transfer buffers but had no sanity

checks on the sizes. This can lead to zero-size-pointer dereferences or

overflowed transfer buffers in ni6501_port_command() and

ni6501_counter_command() if a (malicious) device has smaller max-packet

sizes than expected (or when doing descriptor fuzz testing).

Add the missing sanity checks to probe().