[fix] fix -Wtautological-constant-out-of-range-compare on AIX/xlclang

[mbs-c]

Small fix for a warning in base64_valid the clang-based AIX compiler.

[codecov]

Codecov Report

Merging #44 (24c3f38) into master (9937e6d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #44   +/-   ##
=======================================
  Coverage   97.97%   97.97%           
=======================================
  Files          49       49           
  Lines        8217     8217           
=======================================
  Hits         8051     8051           
  Misses        166      166           

Impacted Files	Coverage Δ
src/c4/base64.cpp	100.00% <100.00%> (ø)

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[mbs-c]

On AIX, char is unsigned by default, which is probably why clang wants the explicit cast. Compiling on another platform with -funsigned-char would probably yield the same warning.

[biojppm]

Thanks!