tree-wide: Switch to autoptr for GOptionContext

We were leaking in a few places that I noticed in an ASAN run.  Also,
this was one of the last non-autoptr cleanup sections we have in
`out:` cleanup sections, making us a lot closer to a potential
full-tree rewrite to `return FALSE`.

Closes: #624
Approved by: jlebon

src/ostree/ot-builtin-cat.c

@@ -61,7 +61,7 @@ cat_one_file (GFile         *f,
 gboolean
 ostree_builtin_cat (int argc, char **argv, GCancellable *cancellable, GError **error)
 {
-  GOptionContext *context;
+  g_autoptr(GOptionContext) context = NULL;
   glnx_unref_object OstreeRepo *repo = NULL;
   gboolean ret = FALSE;
   int i;
@@ -98,7 +98,5 @@ ostree_builtin_cat (int argc, char **argv, GCancellable *cancellable, GError **e
  
   ret = TRUE;
  out:
-  if (context)
-    g_option_context_free (context);
   return ret;
 }