thumbnail: Fix use-after-free when getting a preview icon

g_file_info_get_attribute_object() is transfer none, so when getting a preview GIcon from a gvfs-backed file that supports it, we need to reference the preview otherwise we might crash.

==19044== Invalid read of size 8
==19044==    at 0x48607E7: get_preview_thumbnail (gnome-desktop-thumbnail.c:978)
==19044==    by 0x48607E7: gnome_desktop_thumbnail_factory_generate_thumbnail (gnome-desktop-thumbnail.c:1058)
==19044==    by 0x401181: main (test-desktop-thumbnail.c:51)
==19044==  Address 0x700f750 is 0 bytes inside a block of size 40 free'd
==19044==    at 0x4839A0C: free (vg_replace_malloc.c:530)
==19044==    by 0x48DFCD0: g_type_free_instance (gtype.c:1943)
==19044==    by 0x4E7F7B5: _g_file_attribute_value_clear (gfileattribute.c:176)
==19044==    by 0x4E83D46: g_file_info_finalize (gfileinfo.c:327)
==19044==    by 0x48C1C61: g_object_unref (gobject.c:3346)
==19044==    by 0x48607D5: get_preview_thumbnail (gnome-desktop-thumbnail.c:974)
==19044==    by 0x48607D5: gnome_desktop_thumbnail_factory_generate_thumbnail (gnome-desktop-thumbnail.c:1058)
==19044==    by 0x401181: main (test-desktop-thumbnail.c:51)
==19044==  Block was alloc'd at
==19044==    at 0x483880B: malloc (vg_replace_malloc.c:299)
==19044==    by 0x4B54F20: g_malloc (gmem.c:99)
==19044==    by 0x4B6C3C2: g_slice_alloc (gslice.c:1024)
==19044==    by 0x4B6C9F8: g_slice_alloc0 (gslice.c:1050)
==19044==    by 0x48DFA33: g_type_create_instance (gtype.c:1846)
==19044==    by 0x48C2397: g_object_new_internal (gobject.c:1805)
==19044==    by 0x48C4113: g_object_new_valist (gobject.c:2128)
==19044==    by 0x48C443B: g_object_new (gobject.c:1648)
==19044==    by 0x7451CF7: g_vfs_icon_new (gvfsicon.c:178)
==19044==    by 0x7451D47: g_vfs_icon_from_tokens (gvfsicon.c:268)
==19044==    by 0x4E8BA45: g_icon_new_from_tokens (gicon.c:381)
==19044==    by 0x4E8BA45: g_icon_new_for_string (gicon.c:462)
==19044==    by 0x7450C5F: _g_dbus_get_file_attribute (gvfsdaemonprotocol.c:300)
==19044==    by 0x7450D26: _g_dbus_get_file_info (gvfsdaemonprotocol.c:340)
==19044==    by 0x867A74C: g_daemon_file_query_info (gdaemonfile.c:830)
==19044==    by 0x486078D: get_preview_thumbnail (gnome-desktop-thumbnail.c:960)
==19044==    by 0x486078D: gnome_desktop_thumbnail_factory_generate_thumbnail (gnome-desktop-thumbnail.c:1058)
==19044==    by 0x401181: main (test-desktop-thumbnail.c:51)
==19044==
==19044== Invalid read of size 8
==19044==    at 0x48607F0: get_preview_thumbnail (gnome-desktop-thumbnail.c:978)
==19044==    by 0x48607F0: gnome_desktop_thumbnail_factory_generate_thumbnail (gnome-desktop-thumbnail.c:1058)
==19044==    by 0x401181: main (test-desktop-thumbnail.c:51)
==19044==  Address 0xaaaaaaaaaaaaaaaa is not stack'd, malloc'd or (recently) free'd

Root-caused by "Just Me"

Closes: #87 (closed)