Prevent accidental aggregates

*  The compiler can provide many non-explicit constructors for
   aggregate types.  This is sometimes desired, but it can
   happen accidentally, resulting in run-time errors.

*  This commit assures that no types are aggregates unless existing
   code is using aggregate initialization.

src/ripple/conditions/impl/utils.h

@@ -45,6 +45,7 @@ namespace der {
 // length octets:
 struct Preamble
 {
+    explicit Preamble() = default;
     std::uint8_t type = 0;
     std::size_t tag = 0;
     std::size_t length = 0;