[geary/geary-0.12] Fix crash when GMime sub-message part doesn't actually contain a message.

[Michael Gratton <mjog src gnome org>]

commit 6de8a5a38e4801e24c407ed396c8827afea2d3a3
Author: Michael James Gratton <mike vee net>
Date:   Wed Feb 7 17:37:29 2018 +1100

    Fix crash when GMime sub-message part doesn't actually contain a message.
    
    Patch courtesy Oliver <ogtifs gmail com>
    
    Bug 769697.

 src/engine/rfc822/rfc822-message.vala |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/src/engine/rfc822/rfc822-message.vala b/src/engine/rfc822/rfc822-message.vala
index a2e2868..6a58430 100644
--- a/src/engine/rfc822/rfc822-message.vala
+++ b/src/engine/rfc822/rfc822-message.vala
@@ -960,7 +960,11 @@ public class Geary.RFC822.Message : BaseObject {
         GMime.MessagePart? messagepart = root as GMime.MessagePart;
         if (messagepart != null) {
             GMime.Message sub_message = messagepart.get_message();
-            messages.add(new Geary.RFC822.Message.from_gmime_message(sub_message));
+            if (sub_message != null) {
+                messages.add(new Geary.RFC822.Message.from_gmime_message(sub_message));
+            } else {
+                warning("Corrupt message, possibly bug 769697");
+            }
         }
     }