Fix lsusb -t potentially not listing all devices

There's a bug in the path-parsing code of add_usb_device(). It never checks for
a NULL terminator. This causes some devices to have a bogus
portnum/parent_portnum which in turn makes them not show in the device tree.

Here are two real-world test-cases (yes, the \0 is intentional):
Shows up: ```add_usb_device("1-1.5.1.4.1\0\n\241M")```
Disappers: ```add_usb_device("1-1.5.1.4.2\0\n0c")```