Eliminate potential undefined behavior (RIPD-1685):

Under certain conditions, we could call `memcpy` or `memcmp` with a null
source pointer. Even when specifying 0 as the amount of data to copy this
could result in undefined behavior under the C and C++ standards.

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/basics/Buffer.h

@@ -221,9 +221,13 @@ public:
 
 inline bool operator==(Buffer const& lhs, Buffer const& rhs) noexcept
 {
-    if (lhs.size () != rhs.size ())
+    if (lhs.size() != rhs.size())
         return false;
-    return !std::memcmp (lhs.data (), rhs.data (), lhs.size ());
+
+    if (lhs.size() == 0)
+        return true;
+
+    return std::memcmp(lhs.data(), rhs.data(), lhs.size()) == 0;
 }
 
 inline bool operator!=(Buffer const& lhs, Buffer const& rhs) noexcept

src/ripple/protocol/STBlob.h

@@ -23,6 +23,7 @@
 #include <ripple/basics/Buffer.h>
 #include <ripple/basics/Slice.h>
 #include <ripple/protocol/STBase.h>
+#include <cassert>
 #include <cstring>
 #include <memory>
 
@@ -42,18 +43,6 @@ public:
     {
     }
 
-    /** Construct with size and initializer.
-        Init will be called as:
-            void(void* data, std::size_t size)
-    */
-    template <class Init>
-    STBlob (SField const& f, std::size_t size,
-            Init&& init)
-        : STBase(f), value_ (size)
-    {
-        init(value_.data(), value_.size());
-    }
-
     STBlob (SField const& f,
             void const* data, std::size_t size)
         : STBase(f), value_ (data, size)
@@ -115,12 +104,6 @@ public:
         s.addVL (value_.data (), value_.size ());
     }
 
-    Buffer const&
-    peekValue () const
-    {
-        return value_;
-    }
-
     STBlob&
     operator= (Slice const& slice)
     {
@@ -141,25 +124,12 @@ public:
         return *this;
     }
 
-    Buffer&
-    peekValue ()
-    {
-        return value_;
-    }
-
     void
     setValue (Buffer&& b)
     {
         value_ = std::move (b);
     }
 
-    void
-    setValue (void const* data, std::size_t size)
-    {
-        value_.alloc (size);
-        std::memcpy(value_.data(), data, size);
-    }
-
     bool
     isEquivalent (const STBase& t) const override;
 

src/ripple/protocol/Serializer.h

@@ -49,14 +49,14 @@ public:
         mData.reserve (n);
     }
 
-    Serializer (void const* data,
-        std::size_t size)
+    Serializer (void const* data, std::size_t size)
     {
+        assert(!data == !size);
+
         mData.resize(size);
-        std::memcpy(mData.data(),
-            reinterpret_cast<
-                unsigned char const*>(
-                    data), size);
+
+        if (size)
+            std::memcpy(mData.data(), data, size);
     }
 
     Slice slice() const noexcept