try to fix usb-related fuzz test crashes
in dissect_usb_interface_descriptor() and
dissect_usb_endpoint_descriptor, the offset should be incremented
depending on the number of bytes we dissected, not based on the len
field (len field==0 would then cause an endless loop)
also fixed one occurrence of old_offset-offset, that's probably a typo,
the values will always be negative...