places-sidebar: don't double unref GMount objects
Fixes a lot of reported crashers with NFS/Samba volumes.
https://bugzilla.gnome.org/show_bug.cgi?id=674659 

diff -Naur a/src/caja-places-sidebar.c b/src/caja-places-sidebar.c
--- a/src/caja-places-sidebar.c	2013-07-18 16:53:18.000000000 +0300
+++ b/src/caja-places-sidebar.c	2013-12-08 15:00:03.267535405 +0200
@@ -793,7 +793,7 @@
         root = g_mount_get_default_location (mount);
 
         if (!g_file_is_native (root)) {
-            network_mounts = g_list_prepend (network_mounts, g_object_ref (mount));
+            network_mounts = g_list_prepend (network_mounts, mount);
             continue;
         }
 
@@ -809,7 +809,6 @@
                                location, mount_uri, last_uri,
                                &last_iter, &select_path);
         g_object_unref (root);
-        g_object_unref (mount);
         g_object_unref (icon);
         g_free (name);
         g_free (mount_uri);