IO#sysread should raise IOError when closed

src/io.c

@@ -636,7 +636,7 @@ mrb_io_sysread(mrb_state *mrb, mrb_value io)
     buf = mrb_str_resize(mrb, buf, maxlen);
   }
 
-  fptr = (struct mrb_io *)mrb_get_datatype(mrb, io, &mrb_io_type);
+  fptr = (struct mrb_io *)io_get_open_fptr(mrb, io);
   ret = read(fptr->fd, RSTRING_PTR(buf), maxlen);
   switch (ret) {
     case 0: /* EOF */

test/io.rb

@@ -235,6 +235,7 @@ assert('IO.sysopen, IO#sysread') do
   end
   io.close
   assert_equal "", io.sysread(0)
+  assert_raise(IOError) { io.sysread(1) }
   io.closed?
 end