[Java] Factor in constant fields when computing block length. Issue aeron-io#528.

Author: mjpt777

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

@@ -168,6 +168,11 @@ public PrimitiveType encodingType()
  */
  public int encodedLength()
  {
+ if (presence() == Presence.CONSTANT)
+ {
+ return 0;
+ }
+
  return encodingType.size();
  }
 

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

@@ -425,7 +425,14 @@ else if (field.type() != null)
  return blockLength;
  }
 
- blockLength = field.computedOffset() + fieldLength;
+ if (field.presence() == Presence.CONSTANT)
+ {
+ blockLength = field.computedOffset();
+ }
+ else
+ {
+ blockLength = field.computedOffset() + fieldLength;
+ }
  }
  }
 
@@ -440,7 +447,7 @@ private static void validateBlockLength(
  handleError(
  node,
  "specified blockLength provides insufficient space " +
- computedBlockLength + " > " + specifiedBlockLength);
+  computedBlockLength + " > " + specifiedBlockLength);
  }
  }
 }