drivers/mediatek/afe: Handle zero-length allocation

The older SOF allocator would return a valid pointer into the heap for
a zero length block, but Zephyr's sys_heap returns a NULL.  That's not
an error and it will free() as a noop.

Signed-off-by: Andy Ross <andyross@google.com>

src/drivers/mediatek/afe/afe-drv.c

@@ -380,7 +380,7 @@ int afe_probe(struct mtk_base_afe *afe)
 	afe->irqs_size = platform->irqs_size;
 	afe->irqs = rzalloc(SOF_MEM_ZONE_RUNTIME_SHARED, 0, SOF_MEM_CAPS_RAM,
 			    sizeof(struct mtk_base_afe_irq) * afe->irqs_size);
-	if (!afe->irqs)
+	if (!afe->irqs && afe->irqs_size)
 		goto err_alloc_dais;
 
 	for (i = 0; i < afe->irqs_size; i++)