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.
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Merged revisions 424096 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 424097 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@424098 65c4cc65-6c06-0410-ace0-fbb531ad65f3