tools: kwbimage: Verify size of v0 image header

Check that extended image header size is not larger than file size.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/tools/kwbimage.c b/tools/kwbimage.c
index 542779e..4709c6d 100644
--- a/tools/kwbimage.c
+++ b/tools/kwbimage.c
@@ -1670,6 +1670,9 @@
 		if (mhdr->ext & 0x1) {
 			struct ext_hdr_v0 *ext_hdr;
 
+			if (header_size + sizeof(*ext_hdr) > image_size)
+				return -FDT_ERR_BADSTRUCTURE;
+
 			ext_hdr = (struct ext_hdr_v0 *)
 				(ptr + sizeof(struct main_hdr_v0));
 			checksum = image_checksum8(ext_hdr,