fix: gateway_balance discrepancy (#1839)

[kuznetsss]

Port of #1839 into 2.3.1.
Fixes #1832.

rippled code:

https://github.com/XRPLF/rippled/blob/develop/src/xrpld/rpc/handlers/GatewayBalances.cpp#L129

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 10 lines in your changes missing coverage. Please review.

Project coverage is 71.94%. Comparing base (47c0a6a) to head (8a418bf).
Report is 3 commits behind head on release/2.3.1.

Files with missing lines	Patch %	Lines
src/rpc/handlers/GatewayBalances.hpp	50.00%	1 Missing and 9 partials ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           release/2.3.1    #1874      +/-   ##
=================================================
- Coverage          71.94%   71.94%   -0.01%     
=================================================
  Files                283      283              
  Lines              12067    12071       +4     
  Branches            6144     6146       +2     
=================================================
+ Hits                8682     8684       +2     
- Misses              1782     1784       +2     
  Partials            1603     1603              

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[godexsoft]

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