MLK-20240-7 fastboot: Add NULL return check

Fix coverity issue CID 1898965: Dereference null return value (NULL_RETURNS)
dereference: Dereferencing a pointer that might be NULL dev_desc when calling
write_backup_gpt_partitions

Signed-off-by: Ye Li <ye.li@nxp.com>

drivers/usb/gadget/f_fastboot.c

@@ -880,6 +880,11 @@ int write_backup_gpt(void)
 		return -1;
 	}
 	dev_desc = blk_get_dev("mmc", mmc_no);
+	if (dev_desc == NULL) {
+		printf("Can't get Block device MMC %d\n",
+			mmc_no);
+		return -ENODEV;
+	}
 
 	/* write backup get partition */
 	if (write_backup_gpt_partitions(dev_desc, interface.transfer_buffer)) {