[easytag] Fix crash when saving FLAC tags

[David King <davidk src gnome org>]

commit ebe269ac7bedf4394f57db8712f8dfd402f2d5e3
Author: David King <amigadave amigadave com>
Date:   Wed Jan 7 09:16:46 2015 +0000

    Fix crash when saving FLAC tags
    
    https://bugzilla.gnome.org/show_bug.cgi?id=742350

 src/tags/id3_tag.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)
---
diff --git a/src/tags/id3_tag.c b/src/tags/id3_tag.c
index b86220c..49f50e9 100644
--- a/src/tags/id3_tag.c
+++ b/src/tags/id3_tag.c
@@ -89,9 +89,15 @@ gchar *
 et_id3tag_get_tpos_from_file_tag (const File_Tag *FileTag)
 {
     GString *gstring;
-    gchar *p;
+    const gchar *p;
 
     gstring = g_string_new ("");
+
+    if (!FileTag->disc_number)
+    {
+        return g_string_free (gstring, FALSE);
+    }
+
     p = FileTag->disc_number;
 
     while (*p)