Silence gcc warning.

Silence gcc warning.

Commit 0b94cd9f caused a gcc warning:
streaming_replication/pool_worker_child.c: In function 'do_worker_child':
streaming_replication/pool_worker_child.c:281:40: warning: 'watchdog_leader' may be used uninitialized in this function [-Wmaybe-uninitialized]
         if (!pool_config->use_watchdog ||
                                        ^
It seems this only occures in older gcc (e.g. gcc 4.8.5).

Backpatch-thtrough: master branch only as commit 0b94cd9f only applied to master.