Problem: Spup syntax fails for an out-of-range comment mode.
Solution: Add the missing let to the fallback assignment (Qiming zhao).
Setting oneline_comments above 3 raises E492 while loading the syntax
script. Use a valid legacy Vim script assignment to restore the default
value of 2.
Validated with oneline_comments set to 1, 2, 3, 4 and 99, and with the
variable unset. The original script fails with E492 for a value of 4.
The corrected script loads and applies the expected comment mode.
closes: vim/vim#21227
Supported by AI.
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Co-authored-by: Qiming zhao <chemzqm@gmail.com>
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