Problem: base fee conversion could panic on overflow

[yihuang]

Description

do saturated conversion instead.

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Attention: Patch coverage is 73.68421% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 62.56%. Comparing base (00b9e0e) to head (566f06b).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #433      +/-   ##
===========================================
+ Coverage    62.53%   62.56%   +0.02%     
===========================================
  Files          130      130              
  Lines        12495    12512      +17     
===========================================
+ Hits          7814     7828      +14     
- Misses        4135     4138       +3     
  Partials       546      546              

Files	Coverage Δ
x/feemarket/keeper/params.go	48.57% <100.00%> (ø)
types/int.go	55.88% <72.22%> (+26.47%)	⬆️