homed: fix OOM check

Follow-up for f1b6417fea.
Fixes CID#1593167.

src/home/homed-manager-bus.c

@@ -993,7 +993,7 @@ static int method_add_signing_key(sd_bus_message *message, void *userdata, sd_bu
                 return log_error_errno(r, "Failed to convert public key to PEM: %m");
 
         _cleanup_free_ char *fn_copy = strdup(fn);
-        if (!fn)
+        if (!fn_copy)
                 return log_oom();
 
         _cleanup_free_ char *p = path_join("/var/lib/systemd/home/", fn);