Fix: coredump, dereference null variable

[alfred2g]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.15% :warning:

Comparison is base (ee06ce3) 82.40% compared to head (039fcaa) 82.26%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #327 +/- ## ========================================== - Coverage 82.40% 82.26% -0.15% ========================================== Files 20 20 Lines 8728 8732 +4 ========================================== - Hits 7192 7183 -9 - Misses 1536 1549 +13 ``` | [Files Changed](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/327?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs) | Coverage Δ | | |---|---|---| | [source/v5/mqtt5\_to\_mqtt3\_adapter.c](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/327?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL3Y1L21xdHQ1X3RvX21xdHQzX2FkYXB0ZXIuYw==) | `75.90% <100.00%> (-0.19%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/327/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs)

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