Skip UDP address serialization in muxed conn

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 79.41176% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 79.21%. Comparing base (a834f55) to head (2498b40).

Files	Patch %	Lines
udp_muxed_conn.go	78.12%	9 Missing and 5 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #688 +/- ## ======================================= Coverage 79.20% 79.21% ======================================= Files 41 41 Lines 3737 3733 -4 ======================================= - Hits 2960 2957 -3 - Misses 546 547 +1 + Partials 231 229 -2 ``` | [Flag](https://app.codecov.io/gh/pion/ice/pull/688/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/ice/pull/688/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `79.21% <79.41%> (+<0.01%)` | :arrow_up: | | [wasm](https://app.codecov.io/gh/pion/ice/pull/688/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `22.16% <0.00%> (+0.02%)` | :arrow_up: | 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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