notmuch_filenames_create: Take a reference to underlying filename list

The notmuch_filenames_t object might easily outlive the original object
owning the filename list. So take a talloc reference to keep things safe.

lib/filenames.c

@@ -79,6 +79,7 @@ _notmuch_filenames_create (const void *ctx,
 	return NULL;
 
     filenames->iterator = list->head;
+    (void) talloc_reference (filenames, list);
 
     return filenames;
 }