libusb: guard inaccessible usb devices from segfaults

[alexryndin]

Problem

pilot-link can crash while probing USB devices if the Palm device node exists but is not accessible to the current user.

In that case usb_open() may return NULL, but USB_poll() continues into the normal configuration path and later reaches code that assumes a valid libusb handle. On Arch Linux this reproduced as a SIGSEGV
when pilot-xfer -p usb: -l hit a Palm device under /dev/bus/usb/... without read/write access.

Fix

This change makes USB_poll() fail fast for inaccessible devices and cleanly reset state on early exit paths:

• include <unistd.h> for access()
• check /dev/bus/usb/<bus>/<device> before usb_open()
• skip the device when read/write access is missing
• handle usb_open() returning NULL
• clear data->ref and USB_handle on early failure paths
• avoid calling usb_close() on a null handle

Result

Instead of crashing, the USB probe now logs the permission problem and continues scanning devices.

Verification

Reproduced on Arch Linux with a Palm Tungsten E2 over USB:

• before the patch: pilot-xfer -p usb: -l crashed with SIGSEGV after hitting an inaccessible /dev/bus/usb/... node
• after the patch: the same command logs the access/open failure and exits cleanly without a core dump