Protect against dynamic_cast

Items might not be FOOTPRINT, in which case the dynamic_cast will return
nullptr.  Check for this in the subfunction

pcbnew/pcb_expr_evaluator.cpp

@@ -169,6 +169,9 @@ static void insideCourtyard( LIBEVAL::CONTEXT* aCtx, void* self )
             {
                 SHAPE_POLY_SET footprintCourtyard;
 
+                if( !footprint )
+                    return false;
+
                 if( footprint->IsFlipped() )
                     footprintCourtyard = footprint->GetPolyCourtyardBack();
                 else