dfu: mmc: check if mmc device exists in mmc_block_op()

The function mmc_block_op() is the last function before
the physicall data write, but the mmc device pointer is not
checked. If mmc device not exists, then data abort will occur.
To avoid this, first the mmc device pointer is checked.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
[TestHW: Exynos4412-Trats2]

drivers/dfu/dfu_mmc.c

@@ -40,10 +40,16 @@ static int mmc_access_part(struct dfu_entity *dfu, struct mmc *mmc, int part)
 static int mmc_block_op(enum dfu_op op, struct dfu_entity *dfu,
 			u64 offset, void *buf, long *len)
 {
-	struct mmc *mmc = find_mmc_device(dfu->data.mmc.dev_num);
+	struct mmc *mmc;
 	u32 blk_start, blk_count, n = 0;
 	int ret, part_num_bkp = 0;
 
+	mmc = find_mmc_device(dfu->data.mmc.dev_num);
+	if (!mmc) {
+		error("Device MMC %d - not found!", dfu->data.mmc.dev_num);
+		return -ENODEV;
+	}
+
 	/*
 	 * We must ensure that we work in lba_blk_size chunks, so ALIGN
 	 * this value.