threadpool.c: Minor cleanup fixes.

* Fix threadpool_alloc() prototype.

* Add missing off-nominal NULL check of pool in threadpool_alloc().

* searializer_create() does not need to create the object with a lock as
the lock is not used.
........

Merged revisions 424096 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@424097 f38db490-d61c-443f-a65b-d21fe96a405b