Prevent allocation from encoding UDP address

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 79.14%. Comparing base (75092df) to head (757e240).

Files	Patch %	Lines
udp_muxed_conn.go	83.33%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #684 +/- ## ========================================== + Coverage 79.06% 79.14% +0.07% ========================================== Files 41 41 Lines 3741 3745 +4 ========================================== + Hits 2958 2964 +6 + Misses 549 548 -1 + Partials 234 233 -1 ``` | [Flag](https://app.codecov.io/gh/pion/ice/pull/684/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/684/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `79.14% <83.33%> (+0.07%)` | :arrow_up: | | [wasm](https://app.codecov.io/gh/pion/ice/pull/684/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `22.09% <0.00%> (-0.03%)` | :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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[paulwe]

45ns vs 16ns but it saves 2 allocations per packet