Fix TypeError in extraports_* functions

[orf]

Sometimes there is no extraports tag. In this case a TypeError is raised, which is confusing. Either a proper exception (MissingExtraports or something) should be raised, or None returned. This patch does the latter.

[savon-noir]

Hello,

thx for your contribution.

I have two comments:

1. I personally don't like the idea of having multiple returns in one function unless it really improves readability
2. the change proposed breaks the pep8 compat, see travis log: https://travis-ci.org/savon-noir/python-libnmap/jobs/34698826

Could you correct?

Thanks again for your contribution.

Ronald

[orf]

Done, although in the case of extraports_state I think multiple returns do help readability - the alternative would be to have return {'state': _xtrports['state'], 'count': _xtrports['count']} if _xtraports is not None else None which is not as readable as the current patch. But it's your call.

[coveralls]

Coverage decreased (-0.0%) when pulling 7be7f59 on orf:patch-1 into d8cdc0a on savon-noir:master.

[savon-noir]

sorry for the delays. I was quite busy these days. thanks for your contribution! I need to review the code a bit and do some improvement. Then I'll release a new version on pypi with your contribution included.