[gegl] tools: fix segfault in html generator when using clang

[Øyvind Kolås <ok src gnome org>]

commit e1ae5bd9744c269914f8ceb9706dd5d371de8d2e
Author: Øyvind Kolås <pippin gimp org>
Date:   Sun Feb 18 19:57:27 2018 +0100

    tools: fix segfault in html generator when using clang

 tools/operations_html.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/tools/operations_html.c b/tools/operations_html.c
index f67b491..e5f2c37 100644
--- a/tools/operations_html.c
+++ b/tools/operations_html.c
@@ -709,7 +709,7 @@ all:
         g_print ("%s\n", html_name);
         g_file_set_contents (html_name, s->str, -1, NULL);
       }
-        k = k->next;
+      if (k)  k = k->next;
    }
 
   return 0;