Since commit 297c99a5 an invalid source definition XML of a character
device that is used as backend for RNG devices, smartcards and redirdevs
causes crash of the daemon when parsing such a definition.
The device types mentioned above are not a part of a regular character
device but are backends for other types. Thus when parsing such device
NULL is passed as the argument @chr_def. Later when checking the
validity of the definition @chr_def was dereferenced when parsing a UNIX
socket backend with missing path of the socket and crashed the daemon.
Sample offending configuration:
<devices>
...
<rng model='virtio'>
<backend model='egd' type='unix'>
<source mode='bind' service='1024'/>
</backend>
</rng>
</devices>
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1012196
