[Java] Improve validation of valueRef on fields.

Author: mjpt777

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

@@ -102,9 +102,9 @@ public void validate(final Node node)
  else
  {
  final int periodIndex = valueRef.indexOf('.');
- if (-1 == periodIndex)
+ if (periodIndex < 1 || periodIndex == (valueRef.length() - 1))
  {
- handleError(node, "valueRef format not valid for constant Enum: " + valueRef);
+ handleError(node, "valueRef format not valid for constant (enum-name.valid-value-name): " + valueRef);
  }
 
  final String valueRefType = valueRef.substring(0, periodIndex);