osd/OSDMap: respect temp primary without temp acting

be2748c6d540891f2e1a62e7034cb44f7d04bf18 ensured that
if the temp acting mapping contains only CRUSH_ITEM_NONE,
that the acting_primary is left at -1.  However, even if
acting.empty(), we need to respect a temp_primary mapping.
Thus, use _acting_primary unless acting.empty() &&
acting_primary == -1.

Bug introduced in be2748c6d540891f2e1a62e7034cb44f7d04bf18.
Signed-off-by: Samuel Just <sam.just@inktank.com>