reduce len call

[Deneby67]

reduce len call and up perfomance

[codecov-io]

Codecov Report

Merging #1255 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1255      +/-   ##
==========================================
- Coverage   92.56%   92.53%   -0.04%     
==========================================
  Files          20       20              
  Lines        6143     6145       +2     
==========================================
  Hits         5686     5686              
- Misses        457      459       +2

Impacted Files	Coverage Δ
redis/connection.py	82.45% <100%> (+0.04%)	⬆️
redis/_compat.py	85.6% <0%> (-1.61%)	⬇️

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[andymccurdy]

Thanks. We definitely don't want to cache buffer_length as that changes every iteration of the loop. But I've merged the arg_length argument optimization.