gadget: f_thor: check pointers before use in download_tail()

Some pointers in function download_tail() were not checked
before the use. This could possibly cause the data abort.
To avoid this, check if the pointers are not null is added.

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

drivers/usb/gadget/f_thor.c

@@ -205,12 +205,24 @@ static long long int download_head(unsigned long long total,
 
 static int download_tail(long long int left, int cnt)
 {
-	struct dfu_entity *dfu_entity = dfu_get_entity(alt_setting_num);
-	void *transfer_buffer = dfu_get_buf(dfu_entity);
+	struct dfu_entity *dfu_entity;
+	void *transfer_buffer;
 	int ret;
 
 	debug("%s: left: %llu cnt: %d\n", __func__, left, cnt);
 
+	dfu_entity = dfu_get_entity(alt_setting_num);
+	if (!dfu_entity) {
+		error("Alt setting: %d entity not found!\n", alt_setting_num);
+		return -ENOENT;
+	}
+
+	transfer_buffer = dfu_get_buf(dfu_entity);
+	if (!transfer_buffer) {
+		error("Transfer buffer not allocated!");
+		return -ENXIO;
+	}
+
 	if (left) {
 		ret = dfu_write(dfu_entity, transfer_buffer, left, cnt++);
 		if (ret) {