packetpool: signal waiter within lock

Needed for predictable scheduling. From pthread_cond_signal man page:

"The pthread_cond_signal() or pthread_cond_broadcast() functions may
 be called by a thread whether or not it currently owns the mutex that
 threads calling pthread_cond_wait() or pthread_cond_timedwait() have
 associated with the condition variable during their waits; however, if
 predictable scheduling behaviour is required, then that mutex is locked
 by the thread calling pthread_cond_signal() or pthread_cond_broadcast()."

src/tmqh-packetpool.c

@@ -193,8 +193,8 @@ void PacketPoolReturnPacket(Packet *p)
                 my_pool->pending_tail->next = pool->return_stack.head;
                 pool->return_stack.head = my_pool->pending_head;
                 SC_ATOMIC_RESET(pool->return_stack.sync_now);
-                SCMutexUnlock(&pool->return_stack.mutex);
                 SCCondSignal(&pool->return_stack.cond);
+                SCMutexUnlock(&pool->return_stack.mutex);
                 /* Clear the list of pending packets to return. */
                 my_pool->pending_pool = NULL;
                 my_pool->pending_head = NULL;