Make SessionManager::m_active atomic

It is written under m_mutex but read under m_pending_builds_mutex in one place.

src/session/session_manager.cc

@@ -75,7 +75,6 @@ SessionManager::save_resume_download(core::Download* download) {
   {
     std::unique_lock<std::mutex> lock(m_pending_builds_mutex);
 
-    // TODO: This is under the wrong lock.
     if (!m_active)
       throw torrent::internal_error("SessionManager::save_resume_download() called while not active.");
 

src/session/session_manager.h

@@ -1,6 +1,7 @@
 #ifndef RTORRENT_SESSION_SESSION_MANAGER_H
 #define RTORRENT_SESSION_SESSION_MANAGER_H
 
+#include <atomic>
 #include <condition_variable>
 #include <deque>
 #include <future>
@@ -101,7 +102,7 @@ private:
 
   align_cacheline std::mutex   m_mutex;
 
-  bool                         m_active{};
+  std::atomic<bool>            m_active{};
 
   std::deque<SaveRequest>      m_save_requests;
   std::atomic<size_t>          m_save_request_counter{};