PMM-14321: Fix crash when fingerprinting slowlog queries

[idoqo]

PMM-14321

Link to the Feature Build: SUBMODULES-4098

• PMM-14321: fix pmm-agent crashing when querying system variables go-mysql#72

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.70%. Comparing base (bed7077) to head (196c741).
⚠️ Report is 14 commits behind head on v3.

Additional details and impacted files

@@           Coverage Diff           @@
##               v3    #4721   +/-   ##
=======================================
  Coverage   45.69%   45.70%           
=======================================
  Files         363      363           
  Lines       37654    37654           
=======================================
+ Hits        17207    17209    +2     
+ Misses      18790    18789    -1     
+ Partials     1657     1656    -1     

Flag	Coverage Δ
admin	17.92% <ø> (ø)
agent	53.60% <ø> (ø)
managed	46.18% <ø> (+<0.01%)	⬆️
vmproxy	72.09% <ø> (ø)

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