fixed local DoS when UnregisterHandler was called for a not existing handler

Any user with DBUS access could cause a SEGFAULT in tcmu-runner by
running something like this:

dbus-send --system --print-reply --dest=org.kernel.TCMUService1 /org/kernel/TCMUService1/HandlerManager1 org.kernel.TCMUService1.HandlerManager1.UnregisterHandler string:123