mmc: dw_mmc: Fix potential null pointer risk

we previously assumed 'host->slot' could be null,
null pointer judgment should be added

Signed-off-by: Wen Zhiwei <wenzhiwei@kylinos.cn>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Link: https://lore.kernel.org/r/20211229023814.53372-1-wenzhiwei@kylinos.cn
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/dw_mmc.c

@@ -3568,7 +3568,7 @@ int dw_mci_runtime_resume(struct device *dev)
 	mci_writel(host, CTRL, SDMMC_CTRL_INT_ENABLE);
 
 
-	if (host->slot->mmc->pm_flags & MMC_PM_KEEP_POWER)
+	if (host->slot && host->slot->mmc->pm_flags & MMC_PM_KEEP_POWER)
 		dw_mci_set_ios(host->slot->mmc, &host->slot->mmc->ios);
 
 	/* Force setup bus to guarantee available clock output */