<filesystem type='ram' accessmode='passthrough'>
    <source usage='524288' units='KiB'/>
    <target dir='/dev/shm'/>
  </filesystem>

would lead to lxcContainerMountAllFS calling STRPREFIX
on a NLL pointer because it failed to check if fs->src->path
was non-NULL. This is a regression caused by

  commit da665fbd
  Author: Olga Krishtal <okrishtal@virtuozzo.com>
  Date:   Thu Jul 14 16:52:38 2016 +0300

    filesystem: adds possibility to use storage pool as fs source
Signed-off-by: NOlga Krishtal <okrishtal@virtuozzo.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>