[flang] Only check for embox/rebox if defining op is present

[clementval]

Summary

When the base box is a block argument, the defining op is null.
Only try to recover embox/rebox if there is a defining op.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Lower/ConvertExpr.cpp

updateBoxForParentComponent(Fortran::lower::AbstractConverter &converter,
                             fir::ExtendedValue box,
                             const Fortran::lower::SomeExpr &expr) {
   mlir::Location loc = converter.getCurrentLocation();
   auto &builder = converter.getFirOpBuilder();
   mlir::Value boxBase = fir::getBase(box);
   mlir::Operation *op = boxBase.getDefiningOp();
   mlir::Type actualTy = converter.genType(expr);
 
+  if (op) {
     if (auto embox = mlir::dyn_cast<fir::EmboxOp>(op)) {
       auto newBox = builder.create<fir::EmboxOp>(
           loc, fir::BoxType::get(actualTy), embox.getMemref(), embox.getShape(),
           embox.getSlice(), embox.getTypeparams());
       return fir::substBase(box, newBox);
     }
     if (auto rebox = mlir::dyn_cast<fir::ReboxOp>(op)) {
-    auto newBox = builder.create<fir::ReboxOp>(loc, fir::BoxType::get(actualTy),
-                                               rebox.getBox(), rebox.getShape(),
-                                               rebox.getSlice());
+      auto newBox = builder.create<fir::ReboxOp>(
+          loc, fir::BoxType::get(actualTy), rebox.getBox(), rebox.getShape(),
       return fir::substBase(box, newBox);
     }
+  }
 
   mlir::Value empty;
   mlir::ValueRange emptyRange;
   return builder.create<fir::ReboxOp>(loc, fir::BoxType::get(actualTy), boxBase,
                                       /*shape=*/empty,
                                       /*slice=*/empty);
 }