Remove redundant length check in secure_cmp

[jorickert]

See #92

[codecov]

Codecov Report

Merging #93 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage    95.9%   95.92%   +0.01%     
==========================================
  Files          36       36              
  Lines        5596     5594       -2     
==========================================
- Hits         5367     5366       -1     
+ Misses        229      228       -1

Impacted Files	Coverage Δ
src/util/mod.rs	100% <ø> (ø)	⬆️
src/hazardous/kdf/hkdf.rs	98.78% <0%> (-1.22%)	⬇️
src/hazardous/stream/xchacha20.rs	82.83% <0%> (-0.75%)	⬇️
src/hazardous/aead/chacha20poly1305.rs	88.72% <0%> (-0.6%)	⬇️
src/hazardous/aead/xchacha20poly1305.rs	84.86% <0%> (-0.46%)	⬇️
src/hazardous/stream/chacha20.rs	89.5% <0%> (+1.03%)	⬆️

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[brycx]

Thank you @snsmac for your work on this, I really appreciate it.