fix DecCoins constructor

[fedekunze]

Closes: #5408

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[fedekunze]

this is where coins are sorted

[codecov]

Codecov Report

Merging #5410 into master will increase coverage by 0.24%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #5410      +/-   ##
=========================================
+ Coverage   54.55%   54.8%   +0.24%     
=========================================
  Files         311     311              
  Lines       18756   18761       +5     
=========================================
+ Hits        10233   10282      +49     
+ Misses       7744    7701      -43     
+ Partials      779     778       -1

Impacted Files	Coverage Δ
types/dec_coin.go	78.59% <100%> (+15.38%)	⬆️
x/mock/app.go	64.18% <0%> (+1.35%)	⬆️

[alexanderbez]

ACK -- I think it needs an additional test case for invalid coins.

[alessio]

ACKed