check degraded *before* getting obc

to avoid the crazy assert at the to of get_object_context

     0> 2014-02-15 09:10:53.918386 7fa8dbac7700 -1 osd/ReplicatedPG.cc: In function 'ObjectContextRef ReplicatedPG::get_object_context(const hobject_t&, bool, std::map<std::basic_string<char>, ceph::buffer::ptr>*)' thread 7fa8dbac7700 time 2014-02-15 09:10:53.917026
osd/ReplicatedPG.cc: 6580: FAILED assert(attrs || !pg_log.get_missing().is_missing(soid) || (pg_log.get_log().objects.count(soid) && pg_log.get_log().objects.find(soid)->second->op == pg_log_entry_t::LOST_REVERT))