journald: prevent segfault on empty attr/current
    
    getpidcon() might set con to NULL, even when it returned a 0 return
    code[0]. The subsequent strlen(con) will then cause a segfault.
    
    Alternatively the behaviour could also be changed in getpidcon. I
    don't know whether the libselinux folks are comitted to the current
    behaviour, but the getpidcon man page doesn't really make it obvious
    this case could happen.
    
    [0] https://github.com/SELinuxProject/selinux/blob/fb7f35495fbad468d6efa76c5fed727659903038/libselinux/src/procattr.c#L155-L158
    
    (cherry picked from commit ff868eaadecde2568d1e08a375ec8a3b327984fa)