[balsa] balsa-index: Do not dereference NULL mailbox-node

[Peter Bloomfield <peterb src gnome org>]

commit 995fa8ad7f1fdfe5d0d02cc2ce27e16fb41501cc
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Mon Jul 22 11:24:15 2019 -0400

    balsa-index: Do not dereference NULL mailbox-node

 src/balsa-index.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/src/balsa-index.c b/src/balsa-index.c
index 96a95ddef..9898d8233 100644
--- a/src/balsa-index.c
+++ b/src/balsa-index.c
@@ -2894,7 +2894,8 @@ balsa_index_set_last_use_time(BalsaIndex *bindex)
 {
     g_return_if_fail(BALSA_IS_INDEX(bindex));
 
-    time(&bindex->mailbox_node->last_use);
+    if (bindex->mailbox_node != NULL)
+        time(&bindex->mailbox_node->last_use);
 }
 
 /*