check for nil needle map before compaction sync

When CommitCompact runs concurrently, it sets v.nm = nil under
dataFileAccessLock. CompactByIndex does not hold that lock, so
v.nm.Sync() can hit a nil pointer. Add an early nil check to
return an error instead of crashing.

Fixes #8591

weed/storage/volume_vacuum.go

@@ -117,6 +117,9 @@ func (v *Volume) CompactByIndex(opts *CompactOptions) error {
 	if v.DataBackend == nil {
 		return fmt.Errorf("volume %d backend is empty remote:%v", v.Id, v.HasRemoteFile())
 	}
+	if v.nm == nil {
+		return fmt.Errorf("volume %d needle map is nil", v.Id)
+	}
 	if err := v.DataBackend.Sync(); err != nil {
 		glog.V(0).Infof("compact2 failed to sync volume dat %d: %v", v.Id, err)
 	}