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] fb7f35495f/libselinux/src/procattr.c (L155-L158)

src/journal/journald-context.c

@@ -259,7 +259,7 @@ static int client_context_read_label(
 
                 /* If we got no SELinux label passed in, let's try to acquire one */
 
-                if (getpidcon(c->pid, &con) >= 0) {
+                if (getpidcon(c->pid, &con) >= 0 && con) {
                         free_and_replace(c->label, con);
                         c->label_size = strlen(c->label);
                 }