threadpool: Fix leak in ast_threadpool_serializer_group error path.

ast_threadpool_serializer_group leaks a reference to ser when listener
is allocated but tps is not.  Although listener takes the reference to
ser cleanup functions are not run without tps.

ASTERISK-26191 #close

Change-Id: Ie3ccf69a3f1e676c2ef62a77067c0cb57dc9a585