libusb: idProduct of USB device may be zero

There is no constraint about idProduct in USB spec.  So, pids[i] may be 0 for USB devices.

Change-Id: I19d8974f4e7082e8b7e1f2d33c019ac4e61bc1e2
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1091
Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>