[geary/wip/composer-folks] Make ImapDb.GC's ref to the database strong

[Michael Gratton <mjog src gnome org>]

commit 0f4eda5f45324b353e890b8d02a39e52c2916a89
Author: Michael Gratton <mike vee net>
Date:   Sun Jun 23 12:49:39 2019 +1000

    Make ImapDb.GC's ref to the database strong
    
    This shoud fix Geary.ImapDb.DatabaseTest test failing with a critical:
    
    ```
    geary-CRITICAL **: 05:02:51.688: geary_db_database_get_is_open:
    assertion 'GEARY_DB_IS_DATABASE (self)' failed
    ```
    
    Doing so constitutes a circular ref, but since it is broken once GC is
    complete anyway, it shouldn't be a problem.

 src/engine/imap-db/imap-db-gc.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/engine/imap-db/imap-db-gc.vala b/src/engine/imap-db/imap-db-gc.vala
index 7184ec18..c5773683 100644
--- a/src/engine/imap-db/imap-db-gc.vala
+++ b/src/engine/imap-db/imap-db-gc.vala
@@ -81,7 +81,7 @@ private class Geary.ImapDB.GC {
      */
     public bool is_running { get; private set; default = false; }
 
-    private weak ImapDB.Database db;
+    private ImapDB.Database db;
     private int priority;
 
     public GC(ImapDB.Database db, int priority) {