Fix off-by-one in line number replacement code

[glandium]

Going through the current code fails with:

thread 'tokio-runtime-worker' panicked at src/compiler/c.rs:687:37:
source slice length (3) does not match destination slice length (2)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (1d71d5d) 29.97% compared to head (05eb9cf) 29.95%.

Files	Patch %	Lines
src/compiler/c.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1962      +/-   ##
==========================================
- Coverage   29.97%   29.95%   -0.03%     
==========================================
  Files          51       51              
  Lines       19152    19152              
  Branches     9224     9222       -2     
==========================================
- Hits         5741     5737       -4     
+ Misses       7958     7957       -1     
- Partials     5453     5458       +5     

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