Our ABUSAN CI builder recently flagged how NaN values are handled by the test-only captrunc command when parsing the n argument:

src/mongo/bson/bsonobj.cpp:711:32: runtime error: nan is outside the range of representable values of type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior src/mongo/bson/bsonobj.cpp:711:32 in

Since zero is not an allowed value for this test command, it should be fine to use BSONElement::safeNumberInt() for this parameter.