patch 9.0.0949: crash when unletting a variable while listing variables

Problem:    Crash when unletting a variable while listing variables.
Solution:   Disallow changing a hashtable while going over the entries.
            (closes #11435)
diff --git a/src/terminal.c b/src/terminal.c
index bc36317..d300c59 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -1020,7 +1020,7 @@
 	char *hash_key = alloc(NUMBUFLEN);
 
 	vim_snprintf(hash_key, NUMBUFLEN, "%d", bufnr);
-	hash_add(terminal_bufs, (char_u *)hash_key);
+	hash_add(terminal_bufs, (char_u *)hash_key, "terminal session");
     }
 
     return put_eol(fd);