OSD: don't assume we have the pool in handle_pg_create

The pool may have been removed since the creation message
was sent.  Previously, role would end up as -1 and this
path would be avoided.

Fixes: 7190
Introduced in 268ae82ac3fe7b541c450aae168dcf9fa7294508
Signed-off-by: Samuel Just <sam.just@inktank.com>