Protect courtyard cache regen

The courtyards are referenced by multiple calls and will be rebuilt on
command.  If this happens during zone fill (because the board timestamp
has incremented since the start) multiple threads may generate the cache
at the same time, leaving one with an invalid pointer

Fixes https://gitlab.com/kicad/code/kicad/-/issues/17791

pcbnew/footprint.cpp

@@ -2797,6 +2797,8 @@ std::shared_ptr<SHAPE> FOOTPRINT::GetEffectiveShape( PCB_LAYER_ID aLayer, FLASHI
 
 const SHAPE_POLY_SET& FOOTPRINT::GetCourtyard( PCB_LAYER_ID aLayer ) const
 {
+    std::lock_guard<std::mutex> lock( m_courtyard_cache_mutex );
+
     if( GetBoard() && GetBoard()->GetTimeStamp() > m_courtyard_cache_timestamp )
         const_cast<FOOTPRINT*>( this )->BuildCourtyardCaches();
 

pcbnew/footprint.h

@@ -1050,6 +1050,7 @@ private:
     SHAPE_POLY_SET  m_courtyard_cache_front;  // Note that a footprint can have both front and back
     SHAPE_POLY_SET  m_courtyard_cache_back;   // courtyards populated.
     mutable int     m_courtyard_cache_timestamp;
+    mutable std::mutex      m_courtyard_cache_mutex;
 };
 
 #endif     // FOOTPRINT_H