Guard against multi-threaded access to connectivity rebuild

Fixes https://gitlab.com/kicad/code/kicad/-/issues/19884

pcbnew/connectivity/connectivity_algo.cpp

@@ -228,6 +228,7 @@ void CN_CONNECTIVITY_ALGO::RemoveInvalidRefs()
 
 void CN_CONNECTIVITY_ALGO::searchConnections()
 {
+    std::lock_guard lock( m_mutex );
 #ifdef PROFILE
     PROF_TIMER garbage_collection( "garbage-collection" );
 #endif

pcbnew/connectivity/connectivity_algo.h

@@ -298,6 +298,8 @@ private:
     bool                                                  m_isLocal;
     std::shared_ptr<CONNECTIVITY_DATA>                    m_globalConnectivityData;
 
+    std::mutex m_mutex;
+
     PROGRESS_REPORTER* m_progressReporter = nullptr;
 };