[query] Fix Graphite substr function to handle negative indexes correctly

[yyin-sc]

What this PR does / why we need it:

fix graphite substr() to handle negative index better.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[codecov]

Codecov Report

Merging #3647 (1cd66d5) into master (0ab7b10) will decrease coverage by 1.3%.
The diff coverage is n/a.

❗ Current head 1cd66d5 differs from pull request most recent head 6c729bc. Consider uploading reports for the commit 6c729bc to get more accurate results

@@           Coverage Diff            @@
##           master   #3647     +/-   ##
========================================
- Coverage    57.5%   56.1%   -1.4%     
========================================
  Files         551     551             
  Lines       64031   61876   -2155     
========================================
- Hits        36824   34772   -2052     
+ Misses      24012   23971     -41     
+ Partials     3195    3133     -62     

Flag	Coverage Δ
aggregator	57.1% <ø> (ø)
cluster	∅ <ø> (∅)
collector	58.4% <ø> (ø)
dbnode	60.7% <ø> (-1.7%)	⬇️
m3em	46.4% <ø> (ø)
metrics	19.8% <ø> (ø)
msg	74.4% <ø> (ø)

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[robskillington]

LGTM