Handle shared/frozen strings in IO#sysread

See https://github.com/mruby/mruby/commit/7edbe428caca6814841195a3f2720745cf04353e

src/io.c

@@ -630,8 +630,13 @@ mrb_io_sysread(mrb_state *mrb, mrb_value io)
   if (mrb_nil_p(buf)) {
     buf = mrb_str_new(mrb, NULL, maxlen);
   }
+
+  mrb_str_modify(mrb, RSTRING(buf));
+
   if (RSTRING_LEN(buf) != maxlen) {
     buf = mrb_str_resize(mrb, buf, maxlen);
+  } else {
+    mrb_str_modify(mrb, RSTRING(buf));
   }
 
   fptr = (struct mrb_io *)io_get_open_fptr(mrb, io);

test/io.rb

@@ -250,6 +250,11 @@ assert('IO.sysopen, IO#sysread') do
     io.sysread(10000)
     io.sysread(10000)
   end
+
+  assert_raise RuntimeError do
+    io.sysread(5, "abcde".freeze)
+  end
+
   io.close
   assert_equal "", io.sysread(0)
   assert_raise(IOError) { io.sysread(1) }