journald: prevent segfault on empty attr/current

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