Additional defensive checks

[joe-bowman]

1. Summary

Partially resolves #848

Adds some defensive assertions, notably around use of isZero vs !isPositive when negative values are not permitted.

2.Type of change

• Bug fix (non-breaking change which fixes an issue)

[codecov]

Codecov Report

Merging #878 (b3943f5) into main (bd797bc) will increase coverage by 0.00%.
The diff coverage is 77.33%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #878   +/-   ##
=======================================
  Coverage   61.93%   61.94%           
=======================================
  Files         172      172           
  Lines       14051    14073   +22     
=======================================
+ Hits         8703     8717   +14     
- Misses       4606     4612    +6     
- Partials      742      744    +2     

Flag	Coverage Δ
unittests	61.94% <77.33%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
x/interchainstaking/keeper/delegation.go	92.27% <100.00%> (ø)
x/interchainstaking/keeper/intent.go	76.92% <100.00%> (ø)
x/participationrewards/keeper/distribution.go	68.32% <0.00%> (ø)
x/interchainstaking/keeper/redemptions.go	78.42% <78.57%> (-1.49%)	⬇️