mmc: dw_mmc: Fix NULL pointer dereference

If mrq->sbc is not NULL but data->stop happens to be NULL,
it will lead to NULL pointer dereferencing. Avoid this by
having a NULL check for data->stop.
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <chris@printf.net>

drivers/mmc/host/dw_mmc.c

@@ -1345,7 +1345,7 @@ static void dw_mci_tasklet_func(unsigned long priv)
 
 			if (!err) {
 				if (!data->stop || mrq->sbc) {
-					if (mrq->sbc)
+					if (mrq->sbc && data->stop)
 						data->stop->error = 0;
 					dw_mci_request_end(host, mrq);
 					goto unlock;