[libshumate] network-tile-source: Fix crash when cancellable is NULL

[Marcus Lundblad <mlundblad src gnome org>]

commit 757187617124ef1ac5af47107cdabebf81b51287
Author: James Westman <james jwestman net>
Date:   Tue Mar 23 20:45:15 2021 -0500

    network-tile-source: Fix crash when cancellable is NULL

 shumate/shumate-network-tile-source.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/shumate/shumate-network-tile-source.c b/shumate/shumate-network-tile-source.c
index 5356b52..9377e8d 100644
--- a/shumate/shumate-network-tile-source.c
+++ b/shumate/shumate-network-tile-source.c
@@ -901,7 +901,8 @@ fill_tile (ShumateMapSource *map_source,
   data = g_new0 (FillTileData, 1);
   data->self = g_object_ref (tile_source);
   data->tile = g_object_ref (tile);
-  data->cancellable = g_object_ref (cancellable);
+  if (cancellable)
+    data->cancellable = g_object_ref (cancellable);
 
   shumate_file_cache_get_tile_async (priv->file_cache, tile, cancellable, on_file_cache_get_tile, data);
 }