[evolution-data-server] Avoid possible use after free caused in vee_folder_set_expression()

[Milan Crha <mcrha src gnome org>]

commit a6df5537d1759b9503542b14b6ee45d36d7ea132
Author: Milan Crha <mcrha redhat com>
Date:   Tue Aug 27 18:53:52 2019 +0200

    Avoid possible use after free caused in vee_folder_set_expression()
    
    The private's 'expression' was freed, but it was not reset to NULL,
    when the passed in 'query' was NULL, thus the other code could use
    that freed string.

 src/camel/camel-vee-folder.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)
---
diff --git a/src/camel/camel-vee-folder.c b/src/camel/camel-vee-folder.c
index 383e92385..98374cc02 100644
--- a/src/camel/camel-vee-folder.c
+++ b/src/camel/camel-vee-folder.c
@@ -1121,8 +1121,7 @@ vee_folder_set_expression (CamelVeeFolder *vee_folder,
        }
 
        g_free (vee_folder->priv->expression);
-       if (query)
-               vee_folder->priv->expression = g_strdup (query);
+       vee_folder->priv->expression = g_strdup (query);
 
        vee_folder_rebuild_all (vee_folder, NULL);