[geary: 1/2] PropertyCommand: check for null before calling printf

[Michael Gratton <mjog src gnome org>]

commit 0b62f6eefd5b329b1cae8a3b1611b7ba3093841b
Author: Greg V <greg@unrelenting.technology>
Date:   Wed Jan 9 13:08:58 2019 +0300

    PropertyCommand: check for null before calling printf
    
    On FreeBSD, this would crash in vasprintf_l, because you're not supposed
    to printf(NULL, ...).

 src/client/application/application-command.vala | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)
---
diff --git a/src/client/application/application-command.vala b/src/client/application/application-command.vala
index 3a21027f..1f120246 100644
--- a/src/client/application/application-command.vala
+++ b/src/client/application/application-command.vala
@@ -193,10 +193,18 @@ public class Application.PropertyCommand<T> : Application.Command {
 
         this.object.get(this.property_name, ref this.old_value);
 
-        this.undo_label = undo_label.printf(this.old_value);
-        this.redo_label = redo_label.printf(this.new_value);
-        this.executed_label = executed_label.printf(this.new_value);
-        this.undone_label = undone_label.printf(this.old_value);
+        if (undo_label != null) {
+            this.undo_label = undo_label.printf(this.old_value);
+        }
+        if (redo_label != null) {
+            this.redo_label = redo_label.printf(this.new_value);
+        }
+        if (executed_label != null) {
+            this.executed_label = executed_label.printf(this.new_value);
+        }
+        if (undone_label != null) {
+            this.undone_label = undone_label.printf(this.old_value);
+        }
     }
 
     public async override void execute(GLib.Cancellable? cancellable) {