[balsa] Check for NULL pointer

[Peter Bloomfield <peterb src gnome org>]

commit f20c0f38d9989750fc8219d09b8beeb54b5534ed
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Wed Jun 28 16:25:12 2017 -0400

    Check for NULL pointer
    
        * libbalsa/mailbox.c (lbm_sort): check for NULL prev.

 ChangeLog          |    6 ++++++
 libbalsa/mailbox.c |    3 ++-
 2 files changed, 8 insertions(+), 1 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index e1749b2..0d5194c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 2017-06-28  Peter Bloomfield  <pbloomfield bellsouth net>
 
+       Check for NULL pointer
+
+       * libbalsa/mailbox.c (lbm_sort): check for NULL prev.
+
+2017-06-28  Peter Bloomfield  <pbloomfield bellsouth net>
+
        Handle broken pipe and other errors before writing to pipe
 
        * src/balsa-index.c (pipe_in_watch): handle G_IO_HUP or G_IO_ERR
diff --git a/libbalsa/mailbox.c b/libbalsa/mailbox.c
index 1accc40..91613e9 100644
--- a/libbalsa/mailbox.c
+++ b/libbalsa/mailbox.c
@@ -3508,7 +3508,8 @@ lbm_sort(LibBalsaMailbox * mbox, GNode * parent)
             g_assert(prev == NULL || prev->next == tmp_node);
         prev = tmp_node;
     }
-    prev->next = NULL;
+    if (prev != NULL)
+        prev->next = NULL;
 
     /* Let the world know about our new order */
     new_order = g_new(gint, node_array->len);