lxc: don't try to resolve a NULL path for filesystems

lxc: don't try to resolve a NULL path for filesystems

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

would lead to lxcContainerResolveSymlinks calling
access(NULL) because it failed to check if fs->src->path
was non-NULL. This is a regression caused by

  commit da665fbd4858890fbb3bbf5da2a7b6ca37bb3220
  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: Olga Krishtal <okrishtal@virtuozzo.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>