fix read-error reporting for characters that live in no port

svn: r6031

collects/tests/mzscheme/read.ss

@@ -951,6 +951,11 @@
 (test #\{ syntax-property (read-syntax 'x (open-input-string "{1 2 3}")) 'paren-shape)
 (test #\{ syntax-property (read-syntax 'x (open-input-string "#{1 2}")) 'paren-shape)
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Test read error on a character not in any port
+
+(err/rt-test (read/recursive (open-input-string ";") #\. #f) exn:fail:read?)
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

src/mzscheme/src/error.c

@@ -1323,6 +1323,11 @@ void scheme_read_err(Scheme_Object *port,
 
   show_loc = SCHEME_TRUEP(scheme_get_param(scheme_current_config(), MZCONFIG_ERROR_PRINT_SRCLOC));
 
+  /* Via read/recursive, it's possible that the reader will try to
+     complain about a character that precedes the start of a port.
+     In that case, pos can be 0. */
+  if (!pos) line = col = pos = -1;
+
   if (stxsrc) {
     Scheme_Object *xsrc;