fix nil entry

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x1d340b4]

goroutine 130523 [running]:
github.com/chrislusf/seaweedfs/weed/filer.FileSize(...)
        /code/seaweedfs/weed/filer/filechunks.go:26
github.com/chrislusf/seaweedfs/weed/mount.(*WFS).Lookup(0xc000866d80, 0x1, 0xc002897f40, {0xc004b00980, 0x39}, 0x1ec19e0)
        /code/seaweedfs/weed/mount/weedfs_dir_lookup.go:59 +0x654
github.com/hanwen/go-fuse/v2/fuse.doLookup(0xc00033c000, 0xc00033c000)
        /code/go/pkg/mod/github.com/hanwen/go-fuse/v2@v2.1.0/fuse/opcode.go:333 +0x6b
github.com/hanwen/go-fuse/v2/fuse.(*Server).handleRequest(0xc000ab2420, 0xc00033c000)
        /code/go/pkg/mod/github.com/hanwen/go-fuse/v2@v2.1.0/fuse/server.go:483 +0x1f3
github.com/hanwen/go-fuse/v2/fuse.(*Server).loop(0xc000ab2420, 0x0)
        /code/go/pkg/mod/github.com/hanwen/go-fuse/v2@v2.1.0/fuse/server.go:456 +0x110
created by github.com/hanwen/go-fuse/v2/fuse.(*Server).readRequest
        /code/go/pkg/mod/github.com/hanwen/go-fuse/v2@v2.1.0/fuse/server.go:323 +0x534

weed/mount/weedfs_dir_lookup.go

@@ -55,7 +55,7 @@ func (wfs *WFS) Lookup(cancel <-chan struct{}, header *fuse.InHeader, name strin
 
 	inode := wfs.inodeToPath.Lookup(fullFilePath, localEntry.Mode, len(localEntry.HardLinkId) > 0, localEntry.Inode, true)
 
-	if fh, found := wfs.fhmap.FindFileHandle(inode); found {
+	if fh, found := wfs.fhmap.FindFileHandle(inode); found && fh.entry != nil {
 		glog.V(4).Infof("lookup opened file %s size %d", dirPath.Child(localEntry.Name()), filer.FileSize(fh.entry))
 		localEntry = filer.FromPbEntry(string(dirPath), fh.entry)
 	}