Correct handling of corner-cases involving custom iterators:

- Under some conditions, comparing `ReadViewFwdRange::iterators`
  for equality could derefence an empty `std::unique_ptr` which
  will result in a crash.
- Misuse of the `equal` API could result in a `std::bad_cast`
  exception being thrown from when iterating transactions or
  SLEs from the `OpenView`, `RawStateTable` and `Ledger` classes.

src/ripple/ledger/impl/OpenView.cpp

@@ -45,8 +45,9 @@ public:
     bool
     equal(base_type const& impl) const override
     {
-        auto const& other = dynamic_cast<txs_iter_impl const&>(impl);
-        return iter_ == other.iter_;
+        if (auto const p = dynamic_cast<txs_iter_impl const*>(&impl))
+            return iter_ == p->iter_;
+        return false;
     }
 
     void