Fix 004.watchdog test crash on IBM Z hardware.

Fix 004.watchdog test crash on IBM Z hardware.

When watchdog copies primary node id on the master watchdog node, it
did not consider the case that primary node id on the shared memory
(Req_info->primary_node_id) is remaining in the initial value (-2),
which causes out of range subscript access to backend info
array. Interestingly accessing array[-2] does not crash on intel
architecture but does crash IBM Z hardware. Anyway the reason why the
value remains in -2 is that the regression test is performed in raw
mode. I think the code block handling the primary node id should only
be executed in streaming or logical replication mode.

Bug report and patch provided by gregn123, slightly modified by me.
Mantis bug report: https://www.pgpool.net/mantisbt/view.php?id=614