[C++] Check the length passed to put method for var length strings.

Passing nullptr as src to std::memcpy is undefined behavior even if the
length is zero is undefined behavior.

Author: denizevrenci

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppGenerator.java

@@ -531,9 +531,12 @@ private void generateVarData(
                 indent + "        %3$s lengthFieldValue = %4$s(length);\n" +
                 indent + "        sbePosition(lengthPosition + lengthOfLengthField);\n" +
                 indent + "        std::memcpy(m_buffer + lengthPosition, &lengthFieldValue, sizeof(%3$s));\n" +
-                indent + "        std::uint64_t pos = sbePosition();\n" +
-                indent + "        sbePosition(pos + length);\n" +
-                indent + "        std::memcpy(m_buffer + pos, src, length);\n" +
+                indent + "        if (length != %3$s(0))" +
+                indent + "        {" +
+                indent + "            std::uint64_t pos = sbePosition();\n" +
+                indent + "            sbePosition(pos + length);\n" +
+                indent + "            std::memcpy(m_buffer + pos, src, length);\n" +
+                indent + "        }" +
                 indent + "        return *this;\n" +
                 indent + "    }\n",
                 propertyName,