Clear ratsnest when removing nets

While we can keep the vector for nets, we need to ensure that the data
are cleared to prevent stale pointer access

Fixes https://gitlab.com/kicad/code/kicad/issues/12564

pcbnew/connectivity/connectivity_data.cpp

@@ -209,6 +209,11 @@ void CONNECTIVITY_DATA::RecalculateRatsnest( BOARD_COMMIT* aCommit  )
         for( unsigned int i = prevSize; i < m_nets.size(); i++ )
             m_nets[i] = new RN_NET;
     }
+    else
+    {
+        for( size_t ii = lastNet; ii < m_nets.size(); ++ii )
+            m_nets[ii]->Clear();
+    }
 
     const std::vector<std::shared_ptr<CN_CLUSTER>>& clusters = m_connAlgo->GetClusters();