read-string: return EOF if nothing can be read

R7RS states that there's three possible scenarios for read-string:

- More characters can be read than asked for (return string)
- Less characters can be read than asked for (return string)
- No characters can be read (return EOF)

This commit ensures the last scenario works as intended.

scheme/base.sld

@@ -645,6 +645,8 @@
       (let ((port (if (null? opts)
                       (current-input-port)
                       (car opts))))
+        (if (eof-object? (peek-char port))
+            (eof-object)
             (let loop ((acc '())
                        (i k)
                        (chr #f))
@@ -659,7 +661,7 @@
                (else
                 (loop (if chr (cons chr acc) acc)
                       (- i 1)
-                   (read-char port)))))))
+                      (read-char port))))))))
     (define (flush-output-port . port)
       (if (null? port)
         (Cyc-flush-output-port (current-output-port))