IO#sysread with 0 always return empty string

src/io.c

@@ -625,6 +625,9 @@ mrb_io_sysread(mrb_state *mrb, mrb_value io)
   if (maxlen < 0) {
     return mrb_nil_value();
   }
+  else if (maxlen == 0) {
+    return mrb_str_new(mrb, NULL, maxlen);
+  }
 
   if (mrb_nil_p(buf)) {
     buf = mrb_str_new(mrb, NULL, maxlen);

test/io.rb

@@ -234,6 +234,7 @@ assert('IO.sysopen, IO#sysread') do
     io.sysread(10000)
   end
   io.close
+  assert_equal "", io.sysread(0)
   io.closed?
 end