fix: int64 overflow in module liquidity

[toteki]

Description

closes: #XXXX

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Author Checklist

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Reviewers Checklist

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• confirmed the correct type prefix in the PR title
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[codecov]

Codecov Report

Merging #2072 (a77954e) into main (7f05ad4) will decrease coverage by 4.81%.
The diff coverage is 60.09%.

❗ Current head a77954e differs from pull request most recent head ada0a6a. Consider uploading reports for the commit ada0a6a to get more accurate results

@@            Coverage Diff             @@
##             main    #2072      +/-   ##
==========================================
- Coverage   75.38%   70.58%   -4.81%     
==========================================
  Files         100      126      +26     
  Lines        8025     9580    +1555     
==========================================
+ Hits         6050     6762     +712     
- Misses       1589     2352     +763     
- Partials      386      466      +80     

Impacted Files	Coverage Δ
ante/spam_prevention.go	75.92% <ø> (ø)
util/store/unmarshal.go	0.00% <0.00%> (ø)
x/incentive/codec.go	47.82% <ø> (+9.89%)	⬆️
x/incentive/keeper/hooks.go	0.00% <0.00%> (ø)
x/incentive/keeper/invariants.go	0.00% <0.00%> (ø)
x/incentive/keeper/keeper.go	64.28% <ø> (ø)
x/leverage/keeper/errors.go	100.00% <ø> (ø)
x/leverage/keeper/exchange_rate.go	72.72% <ø> (ø)
x/leverage/keeper/genesis.go	87.23% <ø> (ø)
x/leverage/keeper/grpc_query.go	67.21% <ø> (ø)
... and 80 more

... and 13 files with indirect coverage changes

[robert-zaremba]

pre-approving, let's also include a test to prove the overflow.