OTHER: fix leak on each client connection

The leak was introduced by commit 11d03ee336e97d91f8db39c6e3051d9f6d08ebd3
> BUG(2555): Update g_(thread|mutex|cond)_ calls to GLib 2.32+ API.

which among others converted
    g_thread_create (..., joinable = TRUE, ...)
    g_thread_create (..., joinable = FALSE, ...)

to
    g_thread_new (...)

It is valid only for 'joinable = TRUE' threads
(as their resources are freed by g_thread_join() somewhere.

The leak is seen as a VIRT memory growing 8MB more on amd64
for each client connection.

Running, for example 'xmms2 next' in loop triggered OOM
for a process even on a box with 32GBs of RAM.

This patch fixes leak by dropping reference from thread creator.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>