Fix possible panic on filestore.go (#6912)

Fix to prevent this panic seen in tests:

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xb69322]

goroutine 183 [running]:
github.com/nats-io/nats-server/v2/server.(*fileStore).cleanupOldMeta(0x0)
	server/filestore.go:1984 +0x42
created by github.com/nats-io/nats-server/v2/server.newFileStoreWithCreated.func4 in goroutine 143
	server/filestore.go:519 +0xb1
```

Signed-off-by: Waldemar Quevedo <[email protected]>

Author: neilalexander

server/filestore.go

@@ -516,7 +516,9 @@ func newFileStoreWithCreated(fcfg FileStoreConfig, cfg StreamConfig, created tim
 	// Also make sure we get rid of old idx and fss files on return.
 	// Do this in separate go routine vs inline and at end of processing.
 	defer func() {
-		go fs.cleanupOldMeta()
+		if fs != nil {
+			go fs.cleanupOldMeta()
+		}
 	}()
 
 	// Lock while we do enforcements and removals.