admin: Initialize sysroot to / by default, not garbage from the stack

Oops.

src/ostree/ot-builtin-admin.c

@@ -52,7 +52,7 @@ ostree_builtin_admin (int argc, char **argv, GFile *repo_path, GError **error)
 {
   gboolean ret = FALSE;
   __attribute__((unused)) GCancellable *cancellable = NULL;
-  const char *opt_sysroot;
+  const char *opt_sysroot = "/";
   const char *subcommand_name;
   OstreeAdminCommand *subcommand;
   int subcmd_argc;