Fix infinite loop if thumbnailer is not available

The code in cheese_thumb_view_idle_append_item() in
src/thumbview/cheese-thumb-view.c didn't pop the list of items to
thumbnail if thumbnailing failed.

 #0  0x00007f4a60e55314 in open64 () at /lib64/libc.so.6
 #1  0x00007f4a60de6386 in _IO_file_open () at /lib64/libc.so.6
 #2  0x00007f4a60de655a in __GI__IO_file_fopen () at /lib64/libc.so.6
 #3  0x00007f4a60dd9aad in __fopen_internal () at /lib64/libc.so.6
 #4  0x00007f4a6157a43f in gdk_pixbuf_new_from_file () at /lib64/libgdk_pixbuf-2.0.so.0
 #5  0x00007f4a61e84b3a in gnome_desktop_thumbnail_factory_lookup () at /lib64/libgnome-desktop-3.so.19
 #6  0x000055cef476046f in cheese_thumb_view_idle_append_item ()
 #7  0x00007f4a6124f47b in g_idle_dispatch () at /lib64/libglib-2.0.so.0
 #8  0x00007f4a612537af in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
 #9  0x00007f4a61253b38 in g_main_context_iterate.constprop () at /lib64/libglib-2.0.so.0
 #10 0x00007f4a61253c03 in g_main_context_iteration () at /lib64/libglib-2.0.so.0
 #11 0x00007f4a6146a7ca in g_application_run () at /lib64/libgio-2.0.so.0
 #12 0x000055cef4758547 in _vala_main ()
 #13 0x00007f4a60d8a042 in __libc_start_main () at /lib64/libc.so.6
 #14 0x000055cef47554be in _start ()

 #0  0x00007f4a60ec562d in __strlen_avx2 () at /lib64/libc.so.6
 #1  0x00007f4a61275de8 in g_str_has_suffix () at /lib64/libglib-2.0.so.0
 #2  0x00007f4a618c0072 in icon_name_is_symbolic () at /lib64/libgtk-3.so.0
 #3  0x00007f4a618c00b1 in theme_dir_get_icon_suffix () at /lib64/libgtk-3.so.0
 #4  0x00007f4a618c32ed in theme_lookup_icon () at /lib64/libgtk-3.so.0
 #5  0x00007f4a618c3adf in real_choose_icon () at /lib64/libgtk-3.so.0
 #6  0x00007f4a618c4762 in gtk_icon_theme_lookup_icon_for_scale () at /lib64/libgtk-3.so.0
 #7  0x00007f4a618c5105 in gtk_icon_theme_load_icon_for_scale () at /lib64/libgtk-3.so.0
 #8  0x000055cef47605c9 in cheese_thumb_view_idle_append_item ()
 #9  0x00007f4a6124f47b in g_idle_dispatch () at /lib64/libglib-2.0.so.0
 #10 0x00007f4a612537af in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
 #11 0x00007f4a61253b38 in g_main_context_iterate.constprop () at /lib64/libglib-2.0.so.0
 #12 0x00007f4a61253c03 in g_main_context_iteration () at /lib64/libglib-2.0.so.0
 #13 0x00007f4a6146a7d8 in g_application_run () at /lib64/libgio-2.0.so.0
 #14 0x000055cef4758547 in _vala_main ()
 #15 0x00007f4a60d8a042 in __libc_start_main () at /lib64/libc.so.6
 #16 0x000055cef47554be in _start ()

Closes: #81

src/thumbview/cheese-thumb-view.c

@@ -92,7 +92,7 @@ GtkWidget * cheese_thumb_view_new (void);
 static gboolean
 cheese_thumb_view_idle_append_item (gpointer data)
 {
-  CheeseThumbViewIdleData *item = g_queue_peek_head (data);
+  CheeseThumbViewIdleData *item = g_queue_pop_head (data);
   CheeseThumbView         *thumb_view;
   CheeseThumbViewPrivate  *priv;
 
@@ -119,6 +119,7 @@ cheese_thumb_view_idle_append_item (gpointer data)
   if (!info)
   {
     g_warning ("Invalid filename\n");
+    g_slice_free (CheeseThumbViewIdleData, item);
     return TRUE;
   }
   g_file_info_get_modification_time (info, &mtime);
@@ -167,6 +168,7 @@ cheese_thumb_view_idle_append_item (gpointer data)
     if (error)
     {
       g_warning ("%s", error->message);
+      g_slice_free (CheeseThumbViewIdleData, item);
       return TRUE;
     }
   }
@@ -183,7 +185,6 @@ cheese_thumb_view_idle_append_item (gpointer data)
   g_object_unref (pixbuf);
   g_object_unref (file);
   g_slice_free (CheeseThumbViewIdleData, item);
-  g_queue_pop_head (data);
 
   return TRUE;
 }