crypto: img-hash - Fix variable dereferenced before check 'hdev->req'

Smatch report warning as follows:

drivers/crypto/img-hash.c:366 img_hash_dma_task() warn: variable
dereferenced before check 'hdev->req'

Variable dereferenced should be done after check 'hdev->req',
fix it.

Fixes: d358f1ab ("crypto: img-hash - Add Imagination Technologies hw hash accelerator")
Fixes: 10badea2 ("crypto: img-hash - Fix null pointer exception")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/img-hash.c

@@ -358,12 +358,16 @@ static int img_hash_dma_init(struct img_hash_dev *hdev)
 static void img_hash_dma_task(unsigned long d)
 {
 	struct img_hash_dev *hdev = (struct img_hash_dev *)d;
-	struct img_hash_request_ctx *ctx = ahash_request_ctx(hdev->req);
+	struct img_hash_request_ctx *ctx;
 	u8 *addr;
 	size_t nbytes, bleft, wsend, len, tbc;
 	struct scatterlist tsg;
 
-	if (!hdev->req || !ctx->sg)
+	if (!hdev->req)
+		return;
+
+	ctx = ahash_request_ctx(hdev->req);
+	if (!ctx->sg)
 		return;
 
 	addr = sg_virt(ctx->sg);