sourcemaps: don't map a trailing generated-only segment

[jackyzha0]

Why

The mappings parser detects a segment that has only a generated column by peeking at the next character.

At the end of the mappings string the peek returns an empty string, which is not a separator, so a final generated-only segment is decoded with phantom zero deltas and inherits the source, line, and column of the segment before it.

With --enable-source-maps, stack frames in code after the last mapped segment are then attributed to the last mapped source.

repro.js:

vendor();
function vendor() { throw new Error('boom'); }
//# sourceMappingURL=repro.js.map

repro.js.map:

{"version":3,"sources":["app.ts"],"names":[],"mappings":"AAAA;A"}

$ node --enable-source-maps repro.js
Error: boom
    at vendor (app.ts:1:1)

With this change:

$ node --enable-source-maps repro.js
Error: boom
    at vendor (repro.js:2:27)

What Changed

Check for the end of the string before reading source fields.

Disclaimer: this PR was written with the help of Opus 5.5 but I have personally read, reviewed, and validated the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (e7d8ab5) to head (be06031).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66264      +/-   ##
==========================================
- Coverage   90.30%   90.28%   -0.02%     
==========================================
  Files         789      789              
  Lines      272880   272881       +1     
  Branches    52110    52113       +3     
==========================================
- Hits       246418   246365      -53     
- Misses      16912    16975      +63     
+ Partials     9550     9541       -9     

Files with missing lines	Coverage Δ
lib/internal/source_map/source_map.js	99.48% <100.00%> (+<0.01%)	⬆️

... and 27 files with indirect coverage changes

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