[COMMITTERS] pgsql: Ensure that num_sync is greater than zero in synchronous_standby

Ensure that num_sync is greater than zero in synchronous_standby_names. Previously num_sync could be set to zero and this setting caused an assertion failure. This means that multiple synchronous standbys code should assume that num_sync is greater than zero. Also setting num_sync to zero is nonse

[COMMITTERS] pgsql: Ensure that num_sync is greater than zero in synchronous_standby

Ensure that num_sync is greater than zero in synchronous_standby_names. Previously num_sync could be set to zero and this setting caused an assertion failure. This means that multiple synchronous standbys code should assume that num_sync is greater than zero. Also setting num_sync to zero is nonse