Fix GdkPixbuf leak in gsb_data_account_get_account_icon_pixbuf()
Basically, this patch makes sure that reference counts are coherent,
whether the pixbuf is created from scratch or reused from the account
Code that uses the pixbuf returned from that function has thus been updated
to unref() it once done using the pixbuf.
(cherry picked from commit 72b643412af60fca9b9ff783c50cbd4d61937bc6)