In case of error it also ensures the returned value is NULL.
Else a segfault would occur because the caller (e.g. remsim)
continued on with a free handler.
Change-Id: Ie7f20e3eff03acf77eb08283747ca8e032b9b4c8
USB addresses change every time the device re-enumerates, while the path
reflects the physical topology of USB connections and stays persistent
unless the usb cabling is changed. Let's allow the user to specify the
path instead of the address to uniquely identify a given slot.
Rather than using the first available interface on the first available
device, we now have a "simtrace2-list" program that lists all compatible
interfaces on all configurations of all devices on the system
Kevin Redon
Harald Welte