patch 8.2.2753: Vim9: cannot ignore an item in assignment unpack

Problem:    Vim9: cannot ignore an item in assignment unpack.
Solution:   Allow using an underscore.
diff --git a/src/eval.c b/src/eval.c
index fe9d4ec..787243e 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3514,7 +3514,7 @@
 	{
 	    int	    flags = evalarg == NULL ? 0 : evalarg->eval_flags;
 
-	    if (in_vim9script() && len == 1 && *s == '_')
+	    if (evaluate && in_vim9script() && len == 1 && *s == '_')
 	    {
 		emsg(_(e_cannot_use_underscore_here));
 		ret = FAIL;