threadpool: lock mutex in create

In some contrived case, it might theoretically be the case that
destroy is called from another thread, which would modify the
threads array from two threads.

Coverity 208372

src/common/threadpool.cpp

@@ -71,6 +71,7 @@ void threadpool::recycle() {
 }
 
 void threadpool::create(unsigned int max_threads) {
+  const boost::unique_lock<boost::mutex> lock(mutex);
   boost::thread::attributes attrs;
   attrs.set_stack_size(THREAD_STACK_SIZE);
   max = max_threads ? max_threads : tools::get_max_concurrency();