Add check before minting empty coins

[fedekunze]

The previous function for inflating the supply on the mint module, InflateSupply wasn't checking for neg amounts, nor 0 amounts. The first is already checked on mint due to the fact that we use NewCoin to initialize the coins from the BlockProvision. This PR fixes the second one where a consensus failure was caused if the minted coins where empty.

Note that this check also exists for the two burn functions in staking/keeper/pool.go.

Fix on Gaia: cosmos/gaia#56

cc: @rigelrozanski @alexanderbez

here's the prev InflateSupply:

// when minting new tokens    
func (k Keeper) InflateSupply(ctx sdk.Context, newTokens sdk.Int) {    
    pool := k.GetPool(ctx)    
    pool.NotBondedTokens = pool.NotBondedTokens.Add(newTokens)    
    k.SetPool(ctx, pool)    
}

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added a relevant changelog entry: clog add [section] [stanza] [message]

• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #4681 into master will increase coverage by 0.05%.
The diff coverage is 64.7%.

@@            Coverage Diff             @@
##           master    #4681      +/-   ##
==========================================
+ Coverage   53.99%   54.04%   +0.05%     
==========================================
  Files         271      271              
  Lines       17258    17256       -2     
==========================================
+ Hits         9318     9326       +8     
+ Misses       7255     7251       -4     
+ Partials      685      679       -6

[alexanderbez]

Added some feedback. Also, I believe this warrants a pending log entry...it's kind of a bug fix.

[alexanderbez]

ACK