fix out-of-bounds read when creating a multiplexed tracer

[marten-seemann]

No description provided.

[codecov]

Codecov Report

Merging #2678 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2678      +/-   ##
==========================================
+ Coverage   86.74%   86.78%   +0.04%     
==========================================
  Files         124      124              
  Lines        9897     9897              
==========================================
+ Hits         8585     8589       +4     
+ Misses        978      976       -2     
+ Partials      334      332       -2     

Impacted Files	Coverage Δ
logging/multiplex.go	100.00% <100.00%> (+4.76%)	⬆️

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