fix(worker): default logdir to /tmp when -l not given

logdir was NULL without -l flag, causing stat(NULL)/mkdir(NULL) → EFAULT.
Set default to /tmp after argument parsing, before directory creation.

src/worker/src/main.c

@@ -376,6 +376,10 @@ int main(int argc, char* argv[])
      * without requiring root privileges. */
     check_user();
 
+    if (logdir == NULL) {
+        logdir = strdup("/tmp");
+    }
+
     /* P2.5-5: populate and validate xce_cluster_config_t when --peers given */
     if (peers_str) {
         if (xce_parse_peers(peers_str, &cluster_cfg) != 0) {