GDScript: validate instance before accessing it on error

Make sure the instance is valid before trying to access the script in
after an error happened. If the instance is not valid it's possible that
the script is invalid as well.

Fix #29623

modules/gdscript/gdscript_function.cpp

@@ -1561,14 +1561,14 @@ Variant GDScriptFunction::call(GDScriptInstance *p_instance, const Variant **p_a
 		//error
 		// function, file, line, error, explanation
 		String err_file;
-		if (p_instance && p_instance->script->is_valid() && p_instance->script->path != "")
+		if (p_instance && ObjectDB::instance_validate(p_instance->owner) && p_instance->script->is_valid() && p_instance->script->path != "")
 			err_file = p_instance->script->path;
 		else if (script)
 			err_file = script->path;
 		if (err_file == "")
 			err_file = "<built-in>";
 		String err_func = name;
-		if (p_instance && p_instance->script->is_valid() && p_instance->script->name != "")
+		if (p_instance && ObjectDB::instance_validate(p_instance->owner) && p_instance->script->is_valid() && p_instance->script->name != "")
 			err_func = p_instance->script->name + "." + err_func;
 		int err_line = line;
 		if (err_text == "") {