pg_dump: Set private_date pointer to NULL in callback

pg_dump: Set private_date pointer to NULL in callback

The end callback for ZStandard compression frees the private_data
but didn't set the pointer to NULL after freeing.  This is not a
bug as the code is right now, since nothing is dereferencing the
pointer upon returning from the callback but it is good practice
to do.

Author: Alexander Kuznetsov 
Reviewed-by: Daniel Gustafsson 
Reviewed-by: Ashutosh Bapat 
Reviewed-by: Michael Paquier 
Reviewed-by: Tom Lane 
Discussion: https://postgr.es/m/efaee52b-9550-44ca-8633-ea86076b3283@altlinux.org