[C++]: for aeron-io#714. Use better constant for type than -1 and preserve behavior.

Author: tmontgomery

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

@@ -226,7 +226,7 @@ private static void generateGroupClassHeader(
  indent + " m_bufferLength = bufferLength;\n" +
  indent + " m_blockLength = dimensions.blockLength();\n" +
  indent + " m_count = dimensions.numInGroup();\n" +
- indent + " m_index = -1;\n" +
+ indent + " m_index = std::numeric_limits<std::uint64_t>::max();\n" +
  indent + " m_actingVersion = actingVersion;\n" +
  indent + " m_positionPtr = pos;\n" +
  indent + " *m_positionPtr = *m_positionPtr + %1$d;\n" +
@@ -260,7 +260,7 @@ private static void generateGroupClassHeader(
  indent + " %7$s dimensions(buffer, *pos, bufferLength, actingVersion);\n" +
  indent + " dimensions.blockLength((%1$s)%2$d);\n" +
  indent + " dimensions.numInGroup((%3$s)count);\n" +
- indent + " m_index = -1;\n" +
+ indent + " m_index = std::numeric_limits<std::uint64_t>::max();\n" +
  indent + " m_count = count;\n" +
  indent + " m_blockLength = %2$d;\n" +
  indent + " m_actingVersion = actingVersion;\n" +