fstree: return an error is combined file has incorrect prefix

When we already know that the file is combined (seen the prefix at least
once, isCombined is true) we expect all subsequent entries to be proper
combined ones as well. If they're not (wrong prefix) --- something is wrong
with the file and returning it as is won't help.

Signed-off-by: Roman Khimov <[email protected]>

pkg/local_object_storage/blobstor/fstree/fstree.go

@@ -395,6 +395,9 @@ func extractCombinedObject(id oid.ID, f *os.File) ([]byte, error) {
 		}
 		thisOID, l := parseCombinedPrefix(comBuf)
 		if thisOID == nil {
+			if isCombined {
+				return nil, errors.New("malformed combined file")
+			}
 			st, err := f.Stat()
 			if err != nil {
 				return nil, err