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 44.99%. Comparing base (5ddc655) to head (607a482).

Additional details and impacted files

@@            Coverage Diff             @@
##               v3    #4721      +/-   ##
==========================================
- Coverage   45.74%   44.99%   -0.76%     
==========================================
  Files         364      364              
  Lines       37765    37740      -25     
==========================================
- Hits        17277    16981     -296     
- Misses      18832    19126     +294     
+ Partials     1656     1633      -23     

Flag	Coverage Δ
admin	17.92% <ø> (ø)
agent	50.62% <ø> (-3.10%)	⬇️
vmproxy	72.09% <ø> (ø)

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