remote-refs: Add NULL terminator to options array

Caught by `-fsanitize=undefined`.

Closes: #552
Approved by: jlebon

src/ostree/ot-remote-builtin-refs.c

@@ -29,6 +29,7 @@ static char* opt_cache_dir;
 
 static GOptionEntry option_entries[] = {
   { "cache-dir", 0, 0, G_OPTION_ARG_STRING, &opt_cache_dir, "Use custom cache dir", NULL },
+  { NULL }
 };
 
 gboolean