fix single-argument write-byte and write-char

Repairs a mistake in 8e7792d

Closes PR 15363

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

@@ -1849,6 +1849,20 @@
  resolve-path
  "C:https://testing-root////testing-dir\\\\testing-file")]))
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Make sure `write-byte` and `write-char` don't try to test
+;; a non-supplied argument:
+
+(parameterize ([current-output-port (open-output-string)])
+ (let ([s (if (zero? (random 1)) "a" "b")])
+ (string-append s s) ; causes a clear operation on the runstack for second argument
+ (write-byte 65)))
+
+(parameterize ([current-output-port (open-output-string)])
+ (let ([s (if (zero? (random 1)) "a" "b")])
+ (string-append s s) ; causes a clear operation on the runstack for second argument
+ (write-char #\A)))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

racket/src/racket/src/portfun.c

@@ -4171,6 +4171,7 @@ write_byte (int argc, GC_CAN_IGNORE Scheme_Object *argv[]) XFORM_ASSERT_NO_CONVE
  if (SCHEME_INTP(argv[0])
  && (SCHEME_INT_VAL(argv[0]) >= 0)
  && (SCHEME_INT_VAL(argv[0]) <= 255)
+ && (argc > 1)
  && SCHEME_OUTPUT_PORTP(argv[1])) {
  char buffer[1];
  buffer[0] = SCHEME_INT_VAL(argv[0]);
@@ -4215,6 +4216,7 @@ write_char (int argc, GC_CAN_IGNORE Scheme_Object *argv[]) XFORM_ASSERT_NO_CONVE
  if (argc
  && SCHEME_CHARP(argv[0])
  && (SCHEME_CHAR_VAL(argv[0]) < 128)
+ && (argc > 1)
  && SCHEME_OUTPUT_PORTP(argv[1])) {
  char buffer[1];
  buffer[0] = SCHEME_CHAR_VAL(argv[0]);