fix prop:custom-write printing on structure ports

When a `prop:custom-write` function prints to a given port for
recursive printing, don't reject a port that is a structure port.

Relevant to #1579

pkgs/racket-test-core/tests/racket/print.rktl

@@ -288,6 +288,20 @@
     (define o (open-output-bytes))
     (print ht o)))
 
+;; ----------------------------------------
+;; Check that recursive printing accepts a structure
+;; with the `prop:output-port` property
+
+(let ()
+  (struct p (out) #:property prop:output-port 0)
+  (struct s ()
+          #:property prop:custom-write
+          (lambda (v out mode)
+            (display "ok" (p out))))
+  (define o (open-output-bytes))
+  (write (s) (p o))
+  (test "ok" get-output-string o))
+
 ;; ----------------------------------------
 
 (report-errs)

racket/src/racket/src/print.c

@@ -4235,7 +4235,7 @@ static Scheme_Object *custom_recur(int notdisplay, void *_vec, int argc, Scheme_
   mz_jmp_buf escape, * volatile save;
   volatile intptr_t save_max;
 
-  if (!SCHEME_OUTPORTP(argv[1])) {
+  if (!SCHEME_OUTPUT_PORTP(argv[1])) {
     scheme_wrong_contract((notdisplay > 1)
                           ? "print/recursive"
                           : (notdisplay ? "write/recursive" : "display/recursive"),