osd/OSDMap: populate *primary when pool dne

This fixes a valgrind error from OSD::handle_osd_map where primary is not
initialized and is compared after the call to pg_to_acting_osds().

We are still not distinguishing from "no mapping" to "pool doesn't exist,
no mapping".  That is a somewhat larger change, though.

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