Objecter: potential null pointer access when do pool_snap_list.

Objecter: potential null pointer access when do pool_snap_list. Shall check pool existence first.
Fixes: #13639
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>
(cherry picked from commit 8655416)

src/osdc/Objecter.cc

@@ -1281,6 +1281,8 @@ int Objecter::pool_snap_list(int64_t poolid, vector<uint64_t> *snaps)
   RWLock::RLocker rl(rwlock);
 
   const pg_pool_t *pi = osdmap->get_pg_pool(poolid);
+  if (!pi)
+    return -ENOENT;
   for (map<snapid_t,pool_snap_info_t>::const_iterator p = pi->snaps.begin();
        p != pi->snaps.end();
        ++p) {