guard copyDataBasedOnIndexFile size check against nil needle map

The post-compaction size validation at line 538 accesses
v.nm.ContentSize() and v.nm.DeletedSize(). If CommitCompact has
concurrently set v.nm to nil, this causes a SIGSEGV. Skip the
validation when v.nm is nil since the actual data copy uses local
needle maps (oldNm/newNm) and is unaffected.

Fixes #8591

weed/storage/volume_vacuum.go

@@ -533,7 +533,7 @@ func (v *Volume) copyDataBasedOnIndexFile(opts *CompactOptions) (err error) {
 	if err != nil {
 		return err
 	}
-	if v.Ttl.String() == "" {
+	if v.Ttl.String() == "" && v.nm != nil {
 		dstDatSize, _, err := dstDatBackend.GetStat()
 		if err != nil {
 			return err