storage: skip selinux cleanup when fd not available
    
    When attempting to backport gluster pools to an older versoin
    where there is no VIR_STRDUP, I got a crash from calling
    strdup(,NULL). Rather than relying on the current else branch
    safely doing nothing when there is no fd, it is easier to just
    skip it. While at it, there's no need to explicitly set
    perms.label to NULL after a VIR_FREE().
    
    * src/storage/storage_backend.c
    (virStorageBackendUpdateVolTargetInfoFD): Minor optimization.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>