[geary] Guard against empty envelope.sender list. Bug 772013

[Michael Gratton <mjog src gnome org>]

commit b12aa22e59a7f6d40dad9e7d925d553f5e09fbe5
Author: Timo Kluck <timo kluck booking com>
Date:   Mon Sep 26 17:08:13 2016 +0200

    Guard against empty envelope.sender list. Bug 772013

 src/engine/imap/api/imap-folder.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/engine/imap/api/imap-folder.vala b/src/engine/imap/api/imap-folder.vala
index a15a366..f563ba8 100644
--- a/src/engine/imap/api/imap-folder.vala
+++ b/src/engine/imap/api/imap-folder.vala
@@ -853,7 +853,7 @@ private class Geary.Imap.Folder : BaseObject {
                     email.set_message_subject(envelope.subject);
                     email.set_originators(
                         envelope.from,
-                        envelope.sender.equal_to(envelope.from) ? null : envelope.sender[0],
+                        envelope.sender.equal_to(envelope.from) || envelope.sender.size == 0 ? null : 
envelope.sender[0],
                         envelope.reply_to.equal_to(envelope.from) ? null : envelope.reply_to
                     );
                     email.set_receivers(envelope.to, envelope.cc, envelope.bcc);