Handle timer stop race in agent connectivity check

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 79.12%. Comparing base (0062167) to head (64e7a77).

Files	Patch %	Lines
agent.go	0.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #677 +/- ## ======================================= Coverage 79.11% 79.12% ======================================= Files 41 41 Lines 3740 3741 +1 ======================================= + Hits 2959 2960 +1 Misses 548 548 Partials 233 233 ``` | [Flag](https://app.codecov.io/gh/pion/ice/pull/677/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/677/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `79.12% <0.00%> (+<0.01%)` | :arrow_up: | | [wasm](https://app.codecov.io/gh/pion/ice/pull/677/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `22.12% <0.00%> (-0.01%)` | :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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