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/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