fix initial peek on file that attempts jump past the end

Closes #1938

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

@@ -1104,6 +1104,28 @@
     (let ([x (+ 9 (expt 10 100))])
       (test (list (cons x (add1 x))) regexp-match-peek-positions #"0" p (expt 10 100)))))
 
+;; Make sure that peeking past the end of a
+;; file as a first action gives EOF right away
+(let ([tempfilename (make-temporary-file)])
+  (call-with-output-file*
+   tempfilename
+   #:exists 'truncate
+   (lambda (o) (write-bytes (make-bytes 50 65) o)))
+  (for ([end '(60 50)])
+    (test (list eof eof)
+          call-with-input-file
+          tempfilename
+          (lambda (i)
+            (list (peek-byte i end)
+                  (peek-byte i end))))
+    (test (list eof eof)
+          call-with-input-file
+          tempfilename
+          (lambda (i)
+            (list (peek-bytes 1 end i)
+                  (peek-bytes 1 end i)))))
+  (delete-file tempfilename))
+
 ;;------------------------------------------------------------
 
 ;; Test custom output port

racket/src/racket/src/port.c

@@ -1306,11 +1306,13 @@ intptr_t scheme_get_byte_string_unless(const char *who,
       } else if (v == SCHEME_SPECIAL) {
 	ip->special = NULL;
 	scheme_bad_time_for_special(who, port);
-      } else if (v == skip) {
+      } else if (v > 0) {
 	peek_skip = scheme_bin_minus(peek_skip, scheme_make_integer(skip));
 	/* Ok... ready to continue (if skip == peek_skip) */
-      } else
+      } else {
+        /* This shouldn't happen, but just in case */
 	return 0;
+      }
     }
 
     if (size) {