Fix to inflate: treat EOF as a character, as in the C code.

closes PR 13489.

In the C code, inflate is allowed to peek at least one character
beyond the extent of a deflated byte sequence.  The thread related to
the bug report of PR 13489 documents that deflate can peek beyond EOF.

collects/file/gunzip.rkt

@@ -278,7 +278,8 @@
             (set! buf-pos MAX-LOOKAHEAD))
           (let ([got (peek-bytes-avail! buffer buf-pos #f input-port buf-pos BUFFER-SIZE)])
             (if (eof-object? got)
-                (error 'inflate "unexpected end of file")
+                (begin (bytes-set! buffer buf-pos 255)
+                       (set! buf-max (add1 buf-pos)))
                 (set! buf-max (+ buf-pos got))))
           (READBITS n))
         (let ([v (bytes-ref buffer buf-pos)])