add validation to base64 decoding

[kdaily]

Issue #, if available:

Description of changes:

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[codecov-commenter]

Codecov Report

Merging #30 (996bb6d) into v2 (20d08ae) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##               v2      #30      +/-   ##
==========================================
- Coverage   93.81%   93.70%   -0.12%     
==========================================
  Files         270      351      +81     
  Lines       21417    36171   +14754     
  Branches        0     5202    +5202     
==========================================
+ Hits        20093    33895   +13802     
- Misses       1324     1652     +328     
- Partials        0      624     +624     

Impacted Files	Coverage Δ
awscli/autocomplete/generator.py	82.35% <0.00%> (-17.65%)	⬇️
...i/customizations/lightsail/push_container_image.py	72.58% <0.00%> (-12.91%)	⬇️
awscli/customizations/emr/sshutils.py	60.97% <0.00%> (-12.20%)	⬇️
awscli/customizations/emr/addtags.py	80.00% <0.00%> (-10.00%)	⬇️
awscli/customizations/emr/listclusters.py	82.22% <0.00%> (-8.89%)	⬇️
awscli/customizations/cloudsearch.py	83.33% <0.00%> (-8.34%)	⬇️
awscli/customizations/codedeploy/systems.py	77.98% <0.00%> (-8.26%)	⬇️
awscli/customizations/emr/instancefleetsutils.py	92.30% <0.00%> (-7.70%)	⬇️
awscli/customizations/dynamodb/ddb.py	92.85% <0.00%> (-7.15%)	⬇️
awscli/customizations/ec2/protocolarg.py	93.33% <0.00%> (-6.67%)	⬇️
... and 222 more

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