Fiddle::Function must maintain a reference to the closure

If the first parameter to Fiddle::Function is a closure object (rather
than an interger), `rb_Integer` will cast it to an integer but not
maintain a reference to the closure.  Then if the closure gets GC'd, we
have a segv.  This commit keeps a reference to the original parameter to
initialize so that the object will not be GC'd.

Fixes: https://bugs.ruby-lang.org/issues/13286

Author: tenderlove

ext/fiddle/function.c

@@ -99,6 +99,8 @@ initialize(int argc, VALUE argv[], VALUE self)
     void *cfunc;
 
     rb_scan_args(argc, argv, "31:", &ptr, &args, &ret_type, &abi, &kwds);
+    rb_iv_set(self, "@closure", ptr);
+
     ptr = rb_Integer(ptr);
     cfunc = NUM2PTR(ptr);
     PTR2NUM(cfunc);