osdc/Objecter: fix dereference of NULL pg_pool_t

Make sure we don't dereference a NULL pointer.  Note that we check a
bit further down if the target pool does not exist and return the proper
error.

Bug was reliably reproduced by

 ./ceph_test_rados_api_watch_notify --gtest_filter=LibRadosWatchNotify.WatchNotifyTimeoutTestPP

Signed-off-by: Sage Weil <sage@inktank.com>