Fix panic when adding long comment

[mrsdizzie]

Previous PR #12881 causes out of bounds panic by working on wrong string.

[codecov-commenter]

Codecov Report

Merging #12892 into master will decrease coverage by 0.00%.
The diff coverage is 87.33%.

@@            Coverage Diff             @@
##           master   #12892      +/-   ##
==========================================
- Coverage   43.11%   43.11%   -0.01%     
==========================================
  Files         656      657       +1     
  Lines       72353    72340      -13     
==========================================
- Hits        31194    31186       -8     
+ Misses      36107    36104       -3     
+ Partials     5052     5050       -2     

Impacted Files	Coverage Δ
modules/notification/action/action.go	58.26% <0.00%> (ø)
modules/convert/convert.go	73.00% <50.00%> (-2.37%)	⬇️
modules/convert/issue.go	87.30% <86.20%> (-0.33%)	⬇️
modules/convert/git_commit.go	88.28% <88.28%> (ø)
models/issue_stopwatch.go	70.37% <100.00%> (-2.97%)	⬇️
routers/api/v1/repo/commits.go	57.31% <100.00%> (-14.30%)	⬇️
routers/api/v1/repo/hook.go	26.41% <100.00%> (ø)
routers/api/v1/repo/issue_stopwatch.go	52.63% <100.00%> (ø)
modules/indexer/stats/db.go	60.86% <0.00%> (-8.70%)	⬇️
modules/avatar/avatar.go	50.00% <0.00%> (-4.77%)	⬇️
... and 4 more

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