Fix for exec_async

[barche]

Issue exec_async crashes #174

[ufechner7]

Any idea why it works with Julia 1.6 on Linux, but fails on 1.9 with:

functions.jl: Error During Test at /home/runner/work/QML.jl/QML.jl/test/runtests.jl:14
  Got exception outside of a @test
  LoadError: Storing a null function pointer in a JuliaFunction is not allowed

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (5614e6a) 88.81% compared to head (20cdd00) 89.32%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #185      +/-   ##
==========================================
+ Coverage   88.81%   89.32%   +0.51%     
==========================================
  Files           3        2       -1     
  Lines         438      384      -54     
==========================================
- Hits          389      343      -46     
+ Misses         49       41       -8     

Files	Coverage Δ
src/QML.jl	96.98% <33.33%> (+0.45%)	⬆️

... and 1 file with indirect coverage changes

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