advanced payout: fix star rating overflow

[thelostone-mc]

Description

Issue: https://gitcoincore.slack.com/archives/CAXQ7PT60/p1574199000318000
Fix: https://share.vidyard.com/watch/oRw9gQjxpKH2G9KpJYaDE7?

[codecov]

Codecov Report

Merging #5520 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5520      +/-   ##
==========================================
+ Coverage    29.1%   29.13%   +0.02%     
==========================================
  Files         242      242              
  Lines       20927    20927              
  Branches     3026     3026              
==========================================
+ Hits         6091     6097       +6     
+ Misses      14591    14585       -6     
  Partials      245      245

Impacted Files	Coverage Δ
app/dashboard/embed.py	31.6% <0%> (+3.44%)	⬆️

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