mmc: sdhci: fix NULL dereference in sdhci_request() tuning

Commit 473b095a72a9 ("mmc: sdhci: fix incorrect command used in tuning")
introduced a NULL dereference at resume-time if an SD 3.0 host controller
raises the SDHCI_NEEDS_TUNING flag while no card is inserted.  Seen on an
OLPC XO-4 with sdhci-pxav3, but presumably affects other controllers too.

Signed-off-by: Chris Ball <cjb@laptop.org>
Cc: stable <stable@vger.kernel.org> [3.3+]

drivers/mmc/host/sdhci.c

@@ -1315,16 +1315,19 @@ static void sdhci_request(struct mmc_host *mmc, struct mmc_request *mrq)
 		 */
 		if ((host->flags & SDHCI_NEEDS_RETUNING) &&
 		    !(present_state & (SDHCI_DOING_WRITE | SDHCI_DOING_READ))) {
-			/* eMMC uses cmd21 while sd and sdio use cmd19 */
-			tuning_opcode = mmc->card->type == MMC_TYPE_MMC ?
-				MMC_SEND_TUNING_BLOCK_HS200 :
-				MMC_SEND_TUNING_BLOCK;
-			spin_unlock_irqrestore(&host->lock, flags);
-			sdhci_execute_tuning(mmc, tuning_opcode);
-			spin_lock_irqsave(&host->lock, flags);
-
-			/* Restore original mmc_request structure */
-			host->mrq = mrq;
+			if (mmc->card) {
+				/* eMMC uses cmd21 but sd and sdio use cmd19 */
+				tuning_opcode =
+					mmc->card->type == MMC_TYPE_MMC ?
+					MMC_SEND_TUNING_BLOCK_HS200 :
+					MMC_SEND_TUNING_BLOCK;
+				spin_unlock_irqrestore(&host->lock, flags);
+				sdhci_execute_tuning(mmc, tuning_opcode);
+				spin_lock_irqsave(&host->lock, flags);
+
+				/* Restore original mmc_request structure */
+				host->mrq = mrq;
+			}
 		}
 
 		if (mrq->sbc && !(host->flags & SDHCI_AUTO_CMD23))