scribble lexer returns an empty token

This was found by random testing.

Here's the (non-random) test case that illustrates the bug. The function try should never raise an error (it is a simplification of the checks done by the option in the contract). The string big-s is the one that drdr found and it shrinks to "^|{|\\".

#lang racket
(require syntax-color/scribble-lexer)

(define (try s)
  (define 3ary-lexer (make-scribble-inside-lexer #:command-char #\^))
  (define in (open-input-string s))
  (port-count-lines! in)
  (let loop ([mode #f][offset 0])
    (define-values (txt type paren start end backup new-mode)
      (3ary-lexer in offset mode))
    (cond
      [(equal? type 'eof) #t]
      [else
       (unless (< start end)
         (error 'whoops! "start ~s end ~s" start end))
       (loop new-mode end)])))

;; returns #t if the test case failed, #f if it didn't fail
;; so `s` is helpful when this returns #t
(define (try/bool s)
  (with-handlers ([(λ (x) (and (exn:fail? x)
                               (regexp-match #rx"whoops" (exn-message x))))
                   (λ (x) #t)])
    (try s)
    #f))

(define big-s
  "\"\u0099\"\u009E\u0095Ýäλ|Á5|\u008D)è_)4\u0097(¬{\u009D })Ó|@^|{E cTuÐÆ}\u009E|\\|\u0017)\u0099j\u009A")

(define (get-candidates s)
  (append
   (for/list ([i (in-range (string-length s))])
     (string-append (substring s 0 i)
                    (substring s (+ i 1) (string-length s))))
   (for/list ([i (in-range (string-length s))]
              #:unless (equal? (string-ref s i) #\a))
     (string-append (substring s 0 i)
                    "a"
                    (substring s (+ i 1) (string-length s))))))

(define (shrink s)
  (let loop ([s s])
    (cond
      [(try/bool s)
       (define candidates (get-candidates s))
       (cond
         [(null? candidates) s]
         [else
          (or (for/or ([candidate (in-list candidates)])
                (loop candidate))
              s)])]
      [else #f])))

(shrink big-s)

Start with syntax-color/scribble-lexer and the make-scribble-inside-lexer entry point described in the issue. Run the provided try case with the minimized string "^|{\" and inspect the returned token positions; done means the lexer no longer produces an empty token that makes start fail to be less than end.