[flang][nfc] Add an assert to guard against `nullptr` dereferencing

[awarzynski]

For context, this came up when discussing D119012

[clementval]

LGTM. Please make the change in fir-dev as well. It's getting hard to keep track of the diff while upstreaming so it would be good to sync asap.

[awarzynski]

In D119133#3300860, @clementval wrote:

LGTM. Please make the change in fir-dev as well. It's getting hard to keep track of the diff while upstreaming so it would be good to sync asap.

Thanks, will do!

flang/include/flang/Lower/Bridge.h

return LoweringBridge(ctx, defaultKinds, intrinsics, allCooked, triple,
                           kindMap);
   }
 
   //===--------------------------------------------------------------------===//
   // Getters
   //===--------------------------------------------------------------------===//
 
   mlir::MLIRContext &getMLIRContext() { return context; }
-  mlir::ModuleOp &getModule() { return *module.get(); }
+  mlir::ModuleOp &getModule() {
+    assert(module && "This bridge is missing an MLIR module");
+    return *module.get();
+  }
   const Fortran::common::IntrinsicTypeDefaultKinds &getDefaultKinds() const {
     return defaultKinds;
   }
   const Fortran::evaluate::IntrinsicProcTable &getIntrinsicTable() const {
     return intrinsics;
   }
   const Fortran::parser::AllCookedSources *getCookedSource() const {
     return cooked;