Fix parsing the usbids file

This fixes a regression from 032ca202 where a check on whether the usbids file contained anything but the wrong variable was checked. This is visible on windows when trying to share a USB device as devices won't get named based on the usbids file.

usbids_vendor_info is only initialized to something not NULL after it was checked to be different from NULL which is obviously wrong. The patch wanted to check that the usbids_vendor_count was different from 0 instead.