fix: panic string in staking/val_state_change.go::UnbondingToUnbonded

[danwt]

Fix panic string in staking/val_state_change.go::UnbondingToUnbonded

Description

Closes: #XXXX

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Author Checklist

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please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• 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 #12566 (a170a5e) into main (9c24a58) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main   #12566      +/-   ##
==========================================
- Coverage   65.20%   65.20%   -0.01%     
==========================================
  Files         694      694              
  Lines       71855    71855              
==========================================
- Hits        46853    46851       -2     
- Misses      22371    22374       +3     
+ Partials     2631     2630       -1     

Impacted Files	Coverage Δ
x/staking/keeper/val_state_change.go	64.77% <0.00%> (ø)
crypto/keys/internal/ecdsa/privkey.go	81.13% <0.00%> (-1.89%)	⬇️
x/group/keeper/keeper.go	58.26% <0.00%> (-0.40%)	⬇️