[rygel] server: Prevent critical on upload

[Jens Georg <jensgeorg src gnome org>]

commit 65965a0994f473885a0dfbf3e72385753125a316
Author: Jens Georg <mail jensge org>
Date:   Sat Jun 14 22:30:30 2014 +0200

    server: Prevent critical on upload
    
    Signed-off-by: Jens Georg <mail jensge org>

 src/librygel-server/rygel-media-art-store.vala |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/src/librygel-server/rygel-media-art-store.vala b/src/librygel-server/rygel-media-art-store.vala
index 116be04..453c09c 100644
--- a/src/librygel-server/rygel-media-art-store.vala
+++ b/src/librygel-server/rygel-media-art-store.vala
@@ -59,7 +59,7 @@ public class Rygel.MediaArtStore : GLib.Object {
 
         foreach (var type in types) {
             file = this.get_media_art_file (type, item, simple);
-            if (file.query_exists (null)) {
+            if (file != null && file.query_exists (null)) {
                 break;
             } else {
                 file = null;
@@ -91,9 +91,9 @@ public class Rygel.MediaArtStore : GLib.Object {
         return thumb;
     }
 
-    public File get_media_art_file (string    type,
-                                    MusicItem item,
-                                    bool      simple = false) {
+    public File? get_media_art_file (string    type,
+                                     MusicItem item,
+                                     bool      simple = false) {
         File file;
 
         MediaArt.get_file (item.artist,