[Java] Set size for constant type to zero.

Author: mjpt777

sbe-tool/src/main/java/uk/co/real_logic/sbe/xml/IrGenerator.java

@@ -404,6 +404,7 @@ private void add(final EncodedDataType type, final int offset)
  break;
 
  case CONSTANT:
+ tokenBuilder.size(0);
  encodingBuilder
  .presence(Encoding.Presence.CONSTANT)
  .constValue(type.constVal());