fix(client): fix behavior of `_performShutdown`

[JKRhb]

Should fix #80.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (77e11e3) 83.08% compared to head (b43bafa) 83.08%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #85 +/- ## ======================================= Coverage 83.08% 83.08% ======================================= Files 11 11 Lines 727 727 ======================================= Hits 604 604 Misses 123 123 ``` | [Impacted Files](https://app.codecov.io/gh/JKRhb/dtls2/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann) | Coverage Δ | | |---|---|---| | [lib/src/dtls\_client.dart](https://app.codecov.io/gh/JKRhb/dtls2/pull/85?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann#diff-bGliL3NyYy9kdGxzX2NsaWVudC5kYXJ0) | `79.18% <0.00%> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.