patch 9.2.0618: use-after-free in popup_getoptions() on dict_add() failure

patch 9.2.0618: use-after-free in popup_getoptions() on dict_add() failure

Problem:  use-after-free in popup_getoptions() on dict_add() failure
Solution: Set b to NULL (Yasuhiro Matsumoto).

When dict_add() failed, dictitem_free() unref'd and freed the blob, but
the error cleanup then read the freed blob's refcount and could free it
again. Clear the pointer after dictitem_free().

closes: #20464

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>