Rhombus lexer can error by passing pairs to floor

[samth]

This code https://github.com/racket/rhombus/blob/master/shrubbery-lib/shrubbery/lex-open-close.rkt#L101 can fail when the "column" is not a number but a list (see column+ for places that produce lists). I don't have an obvious reproducer because this happens to me via racket-mode when lexing.