[C++] Initialise encoding when field length is zero. Issue aeron-io#583.

Author: mjpt777

sbe-tool/src/main/cpp/otf/Encoding.h

@@ -161,7 +161,8 @@ class PrimitiveValue
  m_type = type;
  if (0 == valueLength)
  {
- type = PrimitiveType::NONE;
+ m_type = PrimitiveType::NONE;
+ m_size = 0;
  return;
  }
 
@@ -275,6 +276,7 @@ class PrimitiveValue
  default:
  {
  m_type = PrimitiveType::NONE;
+ m_size = 0;
  break;
  }
  }