A minor validation check

[xiazhvera]

Issue #, if available:

Description of changes:
Added a validation check for stop operation.

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 has no change and project coverage change: -0.11 ⚠️

Comparison is base (0e9dc9b) 82.27% compared to head (eb541ee) 82.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #265      +/-   ##
==========================================
- Coverage   82.27%   82.16%   -0.11%     
==========================================
  Files          16       16              
  Lines        7154     7155       +1     
==========================================
- Hits         5886     5879       -7     
- Misses       1268     1276       +8     

Impacted Files	Coverage Δ
source/v5/mqtt5_client.c	88.55% <ø> (ø)
source/client.c	67.80% <0.00%> (-0.59%)	⬇️

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