sharing: Fix crash in media preferences

When XDG user dirs are badly setup, g_get_user_special_dir() will return
NULL, causing us to crash. Use g_strcmp0() instead.

https://bugzilla.gnome.org/show_bug.cgi?id=733489
sharing: Avoid truncated folders list in media sharing

When XDG user dirs are badly setup, and the folders list contains items
such as @VIDEOS@ or @MUSIC@, g_get_user_special_dir() returns NULL and
we would insert NULL into the string array. That would cause the code
reading it to assume the end of the array was reached.

Go through a temporary array instead to build it.

https://bugzilla.gnome.org/show_bug.cgi?id=733489