Closing `DEFAULT` or `NEXT` can crash Ruby

[kojix2]

Yet another bug report based on some rough AI-assisted investigation.

require "fiddle"

Fiddle::Handle::DEFAULT.close
Fiddle::Handle::NEXT.close

Both are pseudo-handles for symbol lookup, not real dlopen() handles. They must not be passed to dlclose().
Expected: raise an exception or do nothing.

[kou]

Users should not do it.

If we accept it, the following code will work:

diff --git a/ext/fiddle/handle.c b/ext/fiddle/handle.c
index 0d03133..34f35e4 100644
--- a/ext/fiddle/handle.c
+++ b/ext/fiddle/handle.c
@@ -72,6 +72,8 @@ rb_fiddle_handle_close(VALUE self)
 {
     struct dl_handle *fiddle_handle;
 
+    rb_check_frozen(self);
+
     TypedData_Get_Struct(self, struct dl_handle, &fiddle_handle_data_type, fiddle_handle);
     if(fiddle_handle->open) {
 	int ret = dlclose(fiddle_handle->ptr);