tools: default_image: Verify header size

Before reading image header, verify that image size is at least size of
the image header.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/default_image.c b/tools/default_image.c
index 0ac3382..ec723f9 100644
--- a/tools/default_image.c
+++ b/tools/default_image.c
@@ -50,6 +50,12 @@
 	struct legacy_img_hdr header;
 	struct legacy_img_hdr *hdr = &header;
 
+	if (image_size < sizeof(struct legacy_img_hdr)) {
+		debug("%s: Bad image size: \"%s\" is no valid image\n",
+		      params->cmdname, params->imagefile);
+		return -FDT_ERR_BADSTRUCTURE;
+	}
+
 	/*
 	 * create copy of header so that we can blank out the
 	 * checksum field for checking - this can't be done