kernel: fatal: Avoid thread api access when no multithreading

Remove access to thread API when multithreading is off.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>

kernel/fatal.c

@@ -98,7 +98,8 @@ void z_fatal_error(unsigned int reason, const z_arch_esf_t *esf)
 	 * appropriate.
 	 */
 	unsigned int key = arch_irq_lock();
-	struct k_thread *thread = k_current_get();
+	struct k_thread *thread = IS_ENABLED(CONFIG_MULTITHREADING) ?
+			k_current_get() : NULL;
 
 	/* twister looks for the "ZEPHYR FATAL ERROR" string, don't
 	 * change it without also updating twister
@@ -179,5 +180,8 @@ void z_fatal_error(unsigned int reason, const z_arch_esf_t *esf)
 	}
 
 	arch_irq_unlock(key);
-	k_thread_abort(thread);
+
+	if (IS_ENABLED(CONFIG_MULTITHREADING)) {
+		k_thread_abort(thread);
+	}
 }