Properly cleanup zstd pipes
Properly cleanup zstd pipes

(cherry picked from commit 0bbc64d5852ab5bde992d025f6297eff6bbc6cbe)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend