fusefrontend: openBackingDir: fix fd leak in error path
fusefrontend: openBackingDir: fix fd leak in error path

Reported by @slackner at
https://github.com/rfjakob/gocryptfs/commit/932efbd4593fe6be6c86f0dafeaea32910b7c246#r31813373
thanks!

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs