reader: fix another here-string problem

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

@@ -219,6 +219,7 @@
 (test "abc" readstr "#<<EOS\nabc\nEOS\n   ")
 (test "abc" readstr "#<<EOS\nabc\nEOS")
 (test "ok\r\nmore\r" readstr "#<<EOS\r\nok\r\nmore\r\nEOS\r")
+(test "" readstr "#<<EOS\nEOS\n")
 (err/rt-test (readstr "#<<EOS\nabc\nEO") exn:fail:read?)
 (err/rt-test (readstr "#<<EOS\r\nabc\nEOS") exn:fail:read?)
 

racket/src/expander/read/string.rkt

@@ -185,8 +185,10 @@
          [(char=? c #\newline) null]
          [else (cons c (loop))]))))
   
-  ;; Get string content
-  (let loop ([terminator full-terminator] [terminator-accum null])
+  ;; Get string content.
+  ;; We just saw a newline that could be considered the start of an
+  ;; immediate `full-terminator`.
+  (let loop ([terminator (cdr full-terminator)] [terminator-accum null])
     (define c (read-char/special in config source))
     (cond
      [(eof-object? c)

racket/src/racket/src/startup.inc

@@ -52177,7 +52177,7 @@ static const char *startup_source =
 "(accum-string-add! accum-str_5 c_88)"
 "(loop_113 full-terminator_0 null)))))))))))))))"
 " loop_113)"
-" full-terminator_0"
+"(cdr full-terminator_0)"
 " null)"
 "(values))))"
 "(let-values(((str_31)"