[easytag] Avoid crash when saving Ogg file with cover art

[David King <davidk src gnome org>]

commit 7542ad103a7280d783de5379f9214d61472e92f9
Author: David King <amigadave amigadave com>
Date:   Sun May 4 09:59:10 2014 +0100

    Avoid crash when saving Ogg file with cover art
    
    https://bugzilla.gnome.org/show_bug.cgi?id=729443

 src/ogg_tag.c |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)
---
diff --git a/src/ogg_tag.c b/src/ogg_tag.c
index df2ee7f..8e88a92 100644
--- a/src/ogg_tag.c
+++ b/src/ogg_tag.c
@@ -1018,6 +1018,7 @@ ogg_tag_write_file_tag (ET_File *ETFile, GError **error)
             guchar *ustring = NULL;
             gsize ustring_len = 0;
             gchar *base64_string;
+            gsize desclen;
             Picture_Format format = Picture_Format_From_Data (pic);
 
             /* According to the specification, only PNG and JPEG images should
@@ -1091,8 +1092,8 @@ ogg_tag_write_file_tag (ET_File *ETFile, GError **error)
             mime = Picture_Mime_Type_String (format);
 
             /* Calculating full length of byte string and allocating. */
-            ustring = g_malloc (4 * 8 + strlen (mime)
-                                + strlen (pic->description) + pic->size);
+            desclen = pic->description ? strlen (pic->description) : 0;
+            ustring = g_malloc (4 * 8 + strlen (mime) + desclen + pic->size);
 
             /* Adding picture type. */
             convert_to_byte_array (pic->type, array);
@@ -1105,11 +1106,11 @@ ogg_tag_write_file_tag (ET_File *ETFile, GError **error)
                                strlen (mime));
 
             /* Adding picture description. */
-            convert_to_byte_array (strlen (pic->description), array);
+            convert_to_byte_array (desclen, array);
             add_to_guchar_str (ustring, &ustring_len, array, 4);
             add_to_guchar_str (ustring, &ustring_len,
                                (guchar *)pic->description,
-                               strlen (pic->description));
+                               desclen);
 
             /* Adding width, height, color depth, indexed colors. */
             convert_to_byte_array (pic->width, array);