[gnome-epub-thumbnailer] mobi: Avoid malformed or missing EXTH header

[Yanko Kaneti <yaneti src gnome org>]

commit 585d4b3cee54fe14eabb767dde72e4226a178ea4
Author: Yanko Kaneti <yaneti declera com>
Date:   Sat May 31 18:21:30 2014 +0300

    mobi: Avoid malformed or missing EXTH header
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1103325

 gnome-mobi-thumbnailer.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/gnome-mobi-thumbnailer.c b/gnome-mobi-thumbnailer.c
index 797192a..41c5d94 100644
--- a/gnome-mobi-thumbnailer.c
+++ b/gnome-mobi-thumbnailer.c
@@ -126,6 +126,11 @@ get_cover_img_num (const char *header)
        len = get_guint32 (header + 20);
        extheader = header + 16 + len;
 
+       if (!g_str_equal (extheader, "EXTH")) {
+               g_warning ("Corrupt or missing EXTH header");
+               return -1;
+       }
+
        num_items = get_guint32 (extheader + 8);
        g_debug ("num extheader items: %d", num_items);
        extheader = extheader + 12;