patch 8.2.1338: Vim9: assigning to script-local variable doesn't check type

Problem:    Vim9: assigning to script-local variable doesn't check type.
Solution:   Use the type. (issue #6591)
diff --git a/src/vim9execute.c b/src/vim9execute.c
index de29988..6e9b53d 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1422,6 +1422,11 @@
 		    dict_T	*dict = tv_dict->vval.v_dict;
 		    dictitem_T	*di;
 
+		    if (dict == NULL)
+		    {
+			emsg(_(e_dictnull));
+			goto on_error;
+		    }
 		    if (key == NULL)
 			key = (char_u *)"";
 		    tv = STACK_TV_BOT(-3);