Port old fuzzing code to go-fuzzing

[tanghaowillow]

Description

port old fuzzing code to go-fuzzing and fix parsing empty time unit causes crash

Reference issue

Fixes #148

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.76% :tada:

Comparison is base (3975f0d) 61.15% compared to head (7823689) 61.91%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #162 +/- ## ========================================== + Coverage 61.15% 61.91% +0.76% ========================================== Files 11 11 Lines 1331 1334 +3 ========================================== + Hits 814 826 +12 + Misses 415 409 -6 + Partials 102 99 -3 ``` | Flag | Coverage Δ | | |---|---|---| | go | `61.91% <100.00%> (+0.76%)` | :arrow_up: | | wasm | `61.91% <100.00%> (+0.76%)` | :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. | [Files Changed](https://app.codecov.io/gh/pion/sdp/pull/162?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | Coverage Δ | | |---|---|---| | [unmarshal.go](https://app.codecov.io/gh/pion/sdp/pull/162?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#diff-dW5tYXJzaGFsLmdv) | `57.07% <100.00%> (+1.17%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/pion/sdp/pull/162/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion)

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[tanghaowillow]

@stv0g Hi, do you have time to have a look at this?

[tanghaowillow]

@stv0g Thanks for the review. I have made another similar pr in the rtcp repo pion/rtcp#161 , would you mind to have a look too?