schedule: fix regression with DRAM execution debugging

Fix a NULL dereference when a __cold function is called before
schedulers are initilised on that core and
CONFIG_COLD_STORE_EXECUTE_DEBUG=y and assertions are enabled.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Author: lyakh

src/include/sof/schedule/schedule.h

@@ -197,6 +197,9 @@ static inline void *scheduler_get_data(uint16_t type)
 	struct schedule_data *sch;
 	struct list_item *slist;
 
+	if (!schedulers)
+		return NULL;
+
 	list_for_item(slist, &schedulers->list) {
 		sch = container_of(slist, struct schedule_data, list);
 		if (type == sch->type)

src/schedule/zephyr_domain.c

@@ -609,7 +609,8 @@ struct ll_schedule_domain *zephyr_domain_init(int clk)
 /* Check if currently running in the LL scheduler thread context */
 bool ll_sch_is_current(void)
 {
-	struct zephyr_domain *zephyr_domain = ll_sch_domain_get_pdata(zephyr_ll_domain());
+	struct ll_schedule_domain *ll_dom = zephyr_ll_domain();
+	struct zephyr_domain *zephyr_domain = ll_dom ? ll_sch_domain_get_pdata(ll_dom) : NULL;
 
 	if (!zephyr_domain)
 		return false;

src/schedule/zephyr_ll.c

@@ -770,5 +770,5 @@ struct ll_schedule_domain *zephyr_ll_domain(void)
 {
 	struct zephyr_ll *ll_sch = scheduler_get_data(SOF_SCHEDULE_LL_TIMER);
 
-	return ll_sch->ll_domain;
+	return ll_sch ? ll_sch->ll_domain : NULL;
 }