[balsa] mailbox-mbox: Guard against NULL pointer

[Peter Bloomfield <peterb src gnome org>]

commit 20ab49a0174bcc797a28364ff76ac2cfc7522be8
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Mon Jul 22 23:45:41 2019 -0400

    mailbox-mbox: Guard against NULL pointer

 libbalsa/mailbox_mbox.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/libbalsa/mailbox_mbox.c b/libbalsa/mailbox_mbox.c
index 9f20b4a54..1e3fa89bf 100644
--- a/libbalsa/mailbox_mbox.c
+++ b/libbalsa/mailbox_mbox.c
@@ -1090,6 +1090,9 @@ libbalsa_mailbox_mbox_close_mailbox(LibBalsaMailbox * mailbox,
     LibBalsaMailboxMbox *mbox = LIBBALSA_MAILBOX_MBOX(mailbox);
     guint len;
 
+    if (mbox->msgno_2_msg_info == NULL)
+        return;
+
     len = mbox->msgno_2_msg_info->len;
     libbalsa_mailbox_mbox_sync(mailbox, expunge);
     if (mbox->msgno_2_msg_info->len != len)