patch 8.2.3053: Vim9: cannot assign to @@ in :def function

Problem:    Vim9: cannot assign to @@ in :def function
Solution:   Handle '@' like '"'.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index bb5ea97..b8a8794 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -5854,7 +5854,8 @@
     }
     else if (*name == '@')
     {
-	if (!valid_yank_reg(name[1], FALSE) || name[1] == '.')
+	if (name[1] != '@'
+			&& (!valid_yank_reg(name[1], FALSE) || name[1] == '.'))
 	{
 	    emsg_invreg(name[1]);
 	    return FAIL;