[C++] Fix call to reset in wrapAndApplyHeader().

The message flyweight should only see buffer length minus the encoded
length of the message header.

Author: denizevrenci

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

@@ -1714,7 +1714,7 @@ private CharSequence generateMessageFlyweightCode(final String className, final
             "        reset(\n" +
             "            buffer + offset + MessageHeader::encodedLength(),\n" +
             "            0,\n" +
-            "            bufferLength,\n" +
+            "            bufferLength - MessageHeader::encodedLength(),\n" +
             "            sbeBlockLength()\n," +
             "            sbeSchemaVersion());\n" +
             "        return *this;\n" +