[gegl] webp-load: guard against NULL error

[Øyvind Kolås <ok src gnome org>]

commit 807305cc36da59eb3c4e5cab8fc6f7d557622a93
Author: Øyvind Kolås <pippin gimp org>
Date:   Sun Mar 12 21:41:57 2017 +0100

    webp-load: guard against NULL error

 operations/external/webp-load.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/operations/external/webp-load.c b/operations/external/webp-load.c
index c525536..7e34608 100644
--- a/operations/external/webp-load.c
+++ b/operations/external/webp-load.c
@@ -205,8 +205,11 @@ prepare (GeglOperation *operation)
       p->stream = gegl_gio_open_input_stream (o->uri, o->path, &p->file, &error);
       if (p->stream == NULL)
         {
-          g_warning ("%s", error->message);
-          g_error_free (error);
+          if (error)
+          {
+            g_warning ("%s", error->message);
+            g_error_free (error);
+          }
           cleanup (operation);
           return;
         }