racket/snip: fix bug in splitting string snips

pkgs/gui-pkgs/gui-test/tests/gracket/editor.rktl

@@ -648,7 +648,17 @@
     (error 'notifications "too weak? ~e" count))
   (unless (<= 0 count2 (/ N 2))
     (error 'notifications "not weak enough? ~e" count2)))
-  
+
+;; ----------------------------------------
+;; make sure splitting a large string snip works:
+
+(void
+ (send (make-object string-snip% (make-string 100000 #\a))
+       split
+       50000
+       (box #f)
+       (box #f)))
+
 ;; ----------------------------------------
 ;; No #<unsafe-undefined> checks on certain class instances
 

pkgs/snip-pkgs/snip-lib/racket/snip/private/snip.rkt

@@ -529,7 +529,7 @@
 
           (let ([s (string-snip-buffer snip)])
             (unless ((string-length s) . >= . position)
-              (set-string-snip-buffer! s (make-string position))))
+              (set-string-snip-buffer! snip (make-string position))))
           
           (string-copy! (string-snip-buffer snip)
                         0