Fix empty, incorrect pool cases

[dongsam]

Fix Depositing to an empty pool results in consensus failure #89
and Trying to buy tokens from an incorrect pool results in chain halting #90 cases

[codecov]

Codecov Report

Merging #91 (52caf9d) into develop (527ea2e) will decrease coverage by 0.14%.
The diff coverage is 71.64%.

@@             Coverage Diff             @@
##           develop      #91      +/-   ##
===========================================
- Coverage    80.47%   80.33%   -0.15%     
===========================================
  Files           18       18              
  Lines         1798     1836      +38     
===========================================
+ Hits          1447     1475      +28     
- Misses         187      192       +5     
- Partials       164      169       +5     

Impacted Files	Coverage Δ
x/liquidity/keeper/liquidity_pool.go	70.08% <71.64%> (+0.33%)	⬆️

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