fix(controller): guard config push against NULL cluster_config on cold boot

Merge the scheduler-join NULL guard from the remote js branch: if the
controller's cluster config is not loaded yet (cold boot / config not
pushed yet), xcesched_push_configs would dereference a NULL cluster_config
and crash the controller (ASan SEGV on scheduler join). Skip the push; the
scheduler receives config on a later join once config is available.

Co-Authored-By: Claude <noreply@anthropic.com>

src/controller/sched.c

@@ -147,6 +147,17 @@ static int xcesched_push_configs(cluster_t* c)
 
     memset(&configdata, 0x0, sizeof(configdata));
 
+    if (c->myconf == NULL || c->myconf->cluster_config == NULL) {
+        /* No cluster config loaded yet (cold boot / config not pushed yet).
+         * Pushing configs now would dereference a NULL cluster_config and
+         * crash the controller (ASan SEGV on scheduler join). Skip; the
+         * scheduler will receive config on a later join once config is
+         * available. */
+        WARN("%s: cluster_config not loaded yet, skipping config push",
+             __func__);
+        return 0;
+    }
+
     configdata.data.conf4sched
         = (xce_conf4sched_t*)calloc(1, sizeof(xce_conf4sched_t));
     if (configdata.data.conf4sched == NULL) {