Unref process_deferred_messages_source when clearing it
Unref process_deferred_messages_source when clearing it

Since g_source_attach adds a ref to a source and we keep the source stored
in a global variable and, therefore, do not unref it after attaching it, we
thus need to unref it when it is being destroyed and we clear the global
variable.

https://bugzilla.gnome.org/show_bug.cgi?id=709625

GNOME/at-spi2-core,GNOME/at-spi2-core,GNOME/at-spi2-core