[tracker] Avoid crash in ttlresource2sgml.c on invalid output path

[Sam Thursfield <sthursfield src gnome org>]

commit 48330f7fbedc44e4ce527e15956367a5bb98d252
Author: Sam Thursfield <sam afuera me uk>
Date:   Sun Feb 26 22:42:00 2017 +0000

    Avoid crash in ttlresource2sgml.c on invalid output path

 docs/tools/ttlresource2sgml.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/docs/tools/ttlresource2sgml.c b/docs/tools/ttlresource2sgml.c
index ae91882..9a24b57 100644
--- a/docs/tools/ttlresource2sgml.c
+++ b/docs/tools/ttlresource2sgml.c
@@ -781,6 +781,11 @@ generate_ontology_class_docs (Ontology    *ontology,
                output_file = g_build_filename (output_dir, class_filename, NULL);
 
                f = fopen (output_file, "w");
+
+               if (f == NULL) {
+                       g_error ("Could not open %s for writing.\n", output_file);
+               }
+
                g_free (class_filename);
                g_free (output_file);
                g_free (shortname);