dmap: fix memory handling surrounding DPAP thumbnails

The reference counting of a DPAP record's thumbnail property was off. This commit makes proper use of g_(byte)_array_ref. Libdmapsharing's dmap-image-connection.c:_handle_mlcl() makes this necessary: it unrefs the thumbnail pointer after passing it to g_object_set(record, ...).

Signed-off-by: W. Michael Petullo mike@flyn.org