java/OFMatchV3Ver13: bugfix for MatchFieldIterator

MatchFieldIterator would return match fields for which the
prerequisites were not met. Then the get() method for those
MatchFields would return null (hiding the fields).
This bug fix makes iterator() behave consistently with
get() / getMasked()
Merge into master from pull request #269:
java/OFMatchV3Ver13: bugfix for MatchFieldIterator (https://github.com/floodlight/loxigen/pull/269)