virThreadPoolFree: Set n(Prio)Workers after the pool is locked

In 78839da I am trying to join the worker threads. However, I can't
sipmly reuse pool->nWorkers (same applies for pool->nPrioWorkers),
because of the following flow that is currently implemented:

1) the main thread executing virThreadPoolFree sets pool->quit = true,
wakes up all the workers and wait on pool->quit_cond.

2) A worker is woken up and see quit request. It immediately jumps of
the while() loop and decrements pool->nWorkers (or pool->nPrioWorkers in
case of priority worker). The last thread signalizes pool->quit_cond.

3) Main thread is woken up, with both pool->nWorkers and
pool->nPrioWorkers being zero.

So there's a need to copy the original value of worker thread counts
into local variables. However, these need to set *after* the check for
pool being NULL (dereferencing a NULL is no no). And for safety they can
be set right after the pool is locked.

Reported-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

src/util/virthreadpool.c

@@ -242,13 +242,15 @@ void virThreadPoolFree(virThreadPoolPtr pool)
     virThreadPoolJobPtr job;
     bool priority = false;
     size_t i;
-    size_t nWorkers = pool->nWorkers;
-    size_t nPrioWorkers = pool->nPrioWorkers;
+    size_t nWorkers;
+    size_t nPrioWorkers;
 
     if (!pool)
         return;
 
     virMutexLock(&pool->mutex);
+    nWorkers = pool->nWorkers;
+    nPrioWorkers = pool->nPrioWorkers;
     pool->quit = true;
     if (pool->nWorkers > 0)
         virCondBroadcast(&pool->cond);