[gegl] warp: avoid crashing with no stroke set

[Øyvind Kolås <ok src gnome org>]

commit af6a707dd745f8b7a70532ac726ca66156c05f71
Author: Øyvind Kolås <pippin gimp org>
Date:   Sun Mar 12 21:41:30 2017 +0100

    warp: avoid crashing with no stroke set

 operations/common/warp.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/operations/common/warp.c b/operations/common/warp.c
index a165ffd..4300349 100644
--- a/operations/common/warp.c
+++ b/operations/common/warp.c
@@ -418,8 +418,9 @@ process (GeglOperation       *operation,
   gulong               i;
   GeglPathList        *event;
 
+  if (!o->stroke)
+    return FALSE;
   priv->buffer = gegl_buffer_dup (input);
-
   event = gegl_path_get_path (o->stroke);
 
   prev = *(event->d.point);