Do not allow more threads than max_threads.

src/main.cpp

@@ -197,7 +197,7 @@ int main(int argc, char *argv[])
 
     for (const auto &filepath : input_files)
     {
-        if (futurepool.size() >= max_threads)
+        while (futurepool.size() >= max_threads)
         {
             futures_cleanup();
         }