Address issues identified by external review:

* RIPD-1617, RIPD-1619, RIPD-1621:
  Verify serialized public keys more strictly before
  using them.

* RIPD-1618:
    * Simplify the base58 decoder logic.
    * Reduce the complexity of the base58 encoder and
      eliminate a potential out-of-bounds memory access.
    * Improve type safety by using an `enum class` to
      enforce strict type checking for token types.

* RIPD-1616:
  Avoid calling `memcpy` with a null pointer even if the
  size is specified as zero, since it results in undefined
  behavior.

Acknowledgements:
Ripple thanks Guido Vranken for responsibly disclosing these
issues.

Bug Bounties and Responsible Disclosures:
We welcome reviews of the rippled code and urge researchers
to responsibly disclose any issues that they may find. For
more on Ripple's Bug Bounty program, please visit:
https://ripple.com/bug-bounty

src/ripple/overlay/impl/OverlayImpl.cpp

@@ -424,7 +424,7 @@ OverlayImpl::add_active (std::shared_ptr<PeerImp> const& peer)
         "activated " << peer->getRemoteAddress() <<
         " (" << peer->id() << ":" <<
         toBase58 (
-            TokenType::TOKEN_NODE_PUBLIC,
+            TokenType::NodePublic,
             peer->getNodePublic()) << ")";
 
     // As we are not on the strand, run() must be called
@@ -610,7 +610,7 @@ OverlayImpl::activate (std::shared_ptr<PeerImp> const& peer)
         "activated " << peer->getRemoteAddress() <<
         " (" << peer->id() <<
         ":" << toBase58 (
-            TokenType::TOKEN_NODE_PUBLIC,
+            TokenType::NodePublic,
             peer->getNodePublic()) << ")";
 
     // We just accepted this peer so we have non-zero active peers