Fix compile warning introduced in r23602

Quote from gnucash-devel:
"I think this introduced a "might be used uninitialized" warning in
src/gnome/top-level.c with my particular version of gcc.

Initializing *file_guid with NULL in gnc_restore_all_state() fixes
the warning, see below."

Author:    Herbert Thoma <herbert.thoma@iis.fraunhofer.de>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23621 57a11ea4-9604-0410-9ed3-97b8803252fd

src/gnome/top-level.c

@@ -241,7 +241,7 @@ static void
 gnc_restore_all_state (gpointer session, gpointer unused)
 {
     GKeyFile *keyfile = NULL;
-    gchar *file_guid;
+    gchar *file_guid = NULL;
     GError *error = NULL;
 
     keyfile = gnc_state_load (session);