fix(resolver): crash when using client upstream (#791)

[0xERR0R]

closes #791

[codecov]

Codecov Report

Base: 93.33% // Head: 93.42% // Increases project coverage by +0.09% 🎉

Coverage data is based on head (4ab7764) compared to base (e08e609).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #793      +/-   ##
===============================================
+ Coverage        93.33%   93.42%   +0.09%     
===============================================
  Files               42       42              
  Lines             4845     4869      +24     
===============================================
+ Hits              4522     4549      +27     
+ Misses             254      252       -2     
+ Partials            69       68       -1     

Impacted Files	Coverage Δ
resolver/parallel_best_resolver.go	100.00% <100.00%> (ø)
resolver/upstream_resolver.go	89.30% <0.00%> (+0.11%)	⬆️
resolver/query_logging_resolver.go	98.68% <0.00%> (+0.18%)	⬆️
redis/redis.go	93.23% <0.00%> (+1.44%)	⬆️

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[kwitsch]

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