[shotwell] piwigo: Prevent a critical

[Jens Georg <jensgeorg src gnome org>]

commit 6afddeff92a119f5198c5c671e8431952bdd36ab
Author: Jens Georg <mail jensge org>
Date:   Mon Oct 3 22:12:01 2016 +0200

    piwigo: Prevent a critical
    
    Signed-off-by: Jens Georg <mail jensge org>

 plugins/shotwell-publishing/PiwigoPublishing.vala |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/plugins/shotwell-publishing/PiwigoPublishing.vala 
b/plugins/shotwell-publishing/PiwigoPublishing.vala
index a7fc0cf..4d92917 100644
--- a/plugins/shotwell-publishing/PiwigoPublishing.vala
+++ b/plugins/shotwell-publishing/PiwigoPublishing.vala
@@ -1437,8 +1437,12 @@ internal class PublishingOptionsPane : Spit.Publishing.DialogPane, Object {
         bool isfirst = true;
         if (publishables != null) {
             foreach (Spit.Publishing.Publishable pub in publishables) {
-                string cur = pub.get_param_string(
+                string? cur = pub.get_param_string(
                     Spit.Publishing.Publishable.PARAM_STRING_EVENTCOMMENT);
+                if (cur == null) {
+                    continue;
+                }
+
                 if (isfirst) {
                     common = cur;
                     isfirst = false;