repair for uri-decode

Certain inputs could cause `uri-decode` to go into a loop.

Patch and supplied by Doug Williams.

Closes PR 14010

pkgs/racket-pkgs/racket-test/tests/net/uri-codec.rkt

@@ -9,6 +9,15 @@
         (uri-decode "%P")  => "%P"
         (uri-decode "λ")   => "λ"
 
+        (uri-decode (string-append
+                     "\u631\u633\u627\u646\u647\u200c\u647\u627\u6cc\u2d\u62e\u648\u62f"
+                     "\u645\u627\u646\u6cc\u2f\u62d\u642\u648\u642\u2d\u628\u634\u631\u2d"
+                     "\u645\u6cc\u62f\u6cc\u627\u2f\u31\u32\u35\u34\u36\u2d"))
+        => (string-append
+            "\u631\u633\u627\u646\u647\u200c\u647\u627\u6cc\u2d\u62e\u648\u62f"
+            "\u645\u627\u646\u6cc\u2f\u62d\u642\u648\u642\u2d\u628\u634\u631\u2d"
+            "\u645\u6cc\u62f\u6cc\u627\u2f\u31\u32\u35\u34\u36\u2d")
+
         (alist->form-urlencoded '([a . "hel+lo \u7238"]))
         => "a=hel%2Blo+%E7%88%B8"
         (form-urlencoded->alist

racket/collects/net/uri-codec.rkt

@@ -184,7 +184,7 @@ See more in PR8831.
                 ;; and not properly encoded -- similar justification to
                 ;; browsers accepting unencoded chars in manually entered URLs.
                 [else (append (bytes->list (string->bytes/utf-8 (string c)))
-                              (internal-decode l))]))))
+                              rest)]))))
   (bytes->string/utf-8 (apply bytes (internal-decode (string->list str)))))
 
 ;; Utility for defining codecs