fix bytes->string error

`thing` is defined as a string on L446, so calling `(bytes->string/utf-8 thing)` on L478 would raise a contract error.

Test case: trying to read `'\\xDDDD;` in an R6RS module

Author: bennn

r6rs-lib/r6rs/private/readtable.rkt

@@ -475,10 +475,9 @@
                                  16)])
                          (unless (or (<= 0 v #xD7FF)
                                      (<= #xE000 v #x10FFFF))
-                           (let ([str (bytes->string/utf-8 thing)])
-                             (raise-read-error
-                              (format "out of range escape: `\\x~a;'" (cadr m))
-                              src line col pos (and pos (string-length str)))))
+                           (raise-read-error
+                            (format "out of range escape: `\\x~a;'" (caddr m))
+                            src line col pos (and pos (string-length thing))))
                          (string->bytes/utf-8 (string (integer->char v))))
                        (loop (cadddr m))))
                 t)))))]

r6rs-test/tests/r6rs/exceptions.sls

@@ -86,6 +86,14 @@
       (test v '(out in out in)))
 
 
+     (test/output
+       (guard (con
+               ((violation? con)
+                (display (condition-message con))
+                'violation))
+              (read (open-string-input-port "\\xDDDD;")))
+       'violation
+       "out of range escape: `\\xDDDD;'")
 
     ;;
     ))