Lock safety.

We're going to grab the lock every time through anyway,
might as well have it for the reads as well as the
writes.

(cherry picked from commit 95ab9f0b50)

pcbnew/tracks_cleaner.cpp

@@ -631,8 +631,9 @@ const std::vector<BOARD_CONNECTED_ITEM*>& TRACKS_CLEANER::getConnectedItems( PCB
                                                          PCB_ZONE_T };
 
     const std::shared_ptr<CONNECTIVITY_DATA>& connectivity = m_brd->GetConnectivity();
+    std::lock_guard                           lock( m_mutex );
 
-    if( std::lock_guard lock( m_mutex ); !m_connectedItemsCache.contains( aTrack ) )
+    if( !m_connectedItemsCache.contains( aTrack ) )
         m_connectedItemsCache[aTrack] = connectivity->GetConnectedItems( aTrack, connectedTypes );
 
     return m_connectedItemsCache.at( aTrack );