Zero-fill private_data when attaching an injection point

Zero-fill private_data when attaching an injection point

InjectionPointAttach() did not initialize the private_data buffer of the
shared memory entry before (perhaps partially) overwriting it.  When the
private data is set to NULL by the caler, the buffer was left
uninitialized.  If set, it could have stale contents.

The buffer is initialized to zero, so as the contents recorded when a
point is attached are deterministic.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0tsGHu2h6YLnVu4HiK05q+gTE_9WVUAqihW2LSscAYS-g@mail.gmail.com
Backpatch-through: 17