qcacld-3.0: Fix unsafe pointer dereference in TWT enable event handler

Fix MAC context NULL pointer dereference in
wma_twt_en_complete_event_handler().

Change-Id: I4b8454032bf44827b174d131145b849935a8a5c4
CRs-Fixed: 2323570

core/wma/src/wma_twt.c

@@ -63,6 +63,10 @@ int wma_twt_en_complete_event_handler(void *handle,
 		WMA_LOGE("Invalid wmi handle for TWT complete");
 		return status;
 	}
+	if (!mac) {
+		WMA_LOGE("Invalid MAC context");
+		return status;
+	}
 	if (wmi_handle->ops->extract_twt_enable_comp_event)
 		status = wmi_handle->ops->extract_twt_enable_comp_event(
 								wmi_handle,