[geary] Fix a crash editing a draft that refers to a message with no message-id.

[Michael Gratton <mjog src gnome org>]

commit a37d8605c2b592c02e602db9d0d849c4b6c2c3c6
Author: Michael James Gratton <mike vee net>
Date:   Tue Apr 17 13:28:02 2018 +1000

    Fix a crash editing a draft that refers to a message with no message-id.

 src/client/composer/composer-widget.vala |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/client/composer/composer-widget.vala b/src/client/composer/composer-widget.vala
index 647d14b..b66cc67 100644
--- a/src/client/composer/composer-widget.vala
+++ b/src/client/composer/composer-widget.vala
@@ -674,7 +674,8 @@ public class ComposerWidget : Gtk.EventBox {
             Gee.Set<Geary.Email> emails = email_map.get_keys();
             Geary.Email? email = null;
             foreach (Geary.Email candidate in emails) {
-                if (candidate.message_id.equal_to(mid)) {
+                if (candidate.message_id != null &&
+                    mid.equal_to(candidate.message_id)) {
                     email = candidate;
                     break;
                 }