Fix PG count change procedure

In previous versions PG histories were calculated incorrectly during
PG count change which led to objects being lost on OSDs not in PG's osd set.

mon/mon.js

@@ -896,6 +896,11 @@ class Mon
                         `PG count for pool ${pool_id} (${pool_cfg.name || 'unnamed'})`+
                         ` changed from: ${old_pg_count} to ${optimize_result.int_pgs.length}`
                     );
+                    // Drop stats
+                    etcd_request.success.push({ requestDeleteRange: {
+                        key: b64(this.etcd_prefix+'/pg/stats/'+pool_id+'/'),
+                        range_end: b64(this.etcd_prefix+'/pg/stats/'+pool_id+'0'),
+                    } });
                 }
                 LPOptimizer.print_change_stats(optimize_result);
                 this.save_new_pgs_txn(etcd_request, pool_id, up_osds, prev_pgs, optimize_result.int_pgs, pg_history);
@@ -1093,11 +1098,14 @@ class Mon
             for (const pg_num in this.state.pg.stats[pool_id])
             {
                 const st = this.state.pg.stats[pool_id][pg_num];
-                for (const k in object_counts)
+                if (st)
                 {
-                    if (st[k+'_count'])
+                    for (const k in object_counts)
                     {
-                        object_counts[k] += BigInt(st[k+'_count']);
+                        if (st[k+'_count'])
+                        {
+                            object_counts[k] += BigInt(st[k+'_count']);
+                        }
                     }
                 }
             }