tools: kwbimage: Fix check for v0 extended header checksum

Extended header checksum for v0 image is present only in the case when
extended header is present. Skip checksum validation if extended header
is not present.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>
diff --git a/tools/kwbimage.c b/tools/kwbimage.c
index 724459f..4d9d818 100644
--- a/tools/kwbimage.c
+++ b/tools/kwbimage.c
@@ -1655,15 +1655,19 @@
 
 	/* Only version 0 extended header has checksum */
 	if (image_version((void *)ptr) == 0) {
-		struct ext_hdr_v0 *ext_hdr;
+		struct main_hdr_v0 *mhdr = (struct main_hdr_v0 *)ptr;
 
-		ext_hdr = (struct ext_hdr_v0 *)
+		if (mhdr->ext & 0x1) {
+			struct ext_hdr_v0 *ext_hdr;
+
+			ext_hdr = (struct ext_hdr_v0 *)
 				(ptr + sizeof(struct main_hdr_v0));
-		checksum = image_checksum8(ext_hdr,
-					   sizeof(struct ext_hdr_v0)
-					   - sizeof(uint8_t));
-		if (checksum != ext_hdr->checksum)
-			return -FDT_ERR_BADSTRUCTURE;
+			checksum = image_checksum8(ext_hdr,
+						   sizeof(struct ext_hdr_v0)
+						   - sizeof(uint8_t));
+			if (checksum != ext_hdr->checksum)
+				return -FDT_ERR_BADSTRUCTURE;
+		}
 	}
 
 	return 0;