src/queue.c: don't deallocate the last block when queue becomes empty.

always keep one block in the queue so that in situations when the queue becomes empty
frequently. performance dosent blow up by calls to malloc/free.

test/t_queue.c

@@ -16,9 +16,6 @@ int main() {
     for(i=0; i < 4; i++) {
         int *c = queue_dequeue(q);
         
-        if (c == NULL)
-            continue;
-            
         assert(c == &map[i]);
     }