Initialize source port and destination port if possible

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.23529% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 80.79%. Comparing base (32ef4a1) to head (90d37b4).

Files	Patch %	Lines
association.go	88.23%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #312 +/- ## ========================================== + Coverage 80.77% 80.79% +0.02% ========================================== Files 49 49 Lines 4135 4150 +15 ========================================== + Hits 3340 3353 +13 - Misses 651 653 +2 Partials 144 144 ``` | [Flag](https://app.codecov.io/gh/pion/sctp/pull/312/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | Coverage Δ | | |---|---|---| | [go](https://app.codecov.io/gh/pion/sctp/pull/312/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `80.79% <88.23%> (+0.02%)` | :arrow_up: | | [wasm](https://app.codecov.io/gh/pion/sctp/pull/312/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `67.03% <47.05%> (-0.10%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#carryforward-flags-in-the-pull-request-comment) to find out more.

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[stv0g]

I am wondering, does this break backward compatibility with existing applications which pass a net.Conn?

[hugoArregui]

@stv0g I don't think so because I think peers are either not validating the port, or are going to validate the port correctly now. But this is me just playing with the protocol a little bit so maybe there are some cases I'm missing.