avoid slice out of bounds

avoid this problem

2018/09/04 16:27:14 fuse: panic in handler for Write [ID=0x27c0d Node=0x2 Uid=0 Gid=0 Pid=0] 0x1 131072 @10607788032 fl=WriteCache lock=0 ffl=OpenReadOnly: runtime error: slice bounds out of range
goroutine 211141 [running]:
bazil.org/fuse/fs.(*Server).serve.func2(0x10d3e60, 0xc00014be30, 0xc00052fef8, 0xc00052fe77)
	/home/travis/gopath/src/bazil.org/fuse/fs/serve.go:857 +0x1ac
panic(0xe2d080, 0x17f62b0)
	/home/travis/.gimme/versions/go/src/runtime/panic.go:513 +0x1b9
github.com/chrislusf/seaweedfs/weed/filesys.(*ContinuousDirtyPages).saveToStorage(0xc0000aca80, 0x10d7ba0, 0xc0003fcc00, 0xc0005dc000, 0x20000, 0x1000000, 0x276720000, 0xc0003feaa0, 0x0, 0x0)
	/home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/filesys/dirty_page.go:142 +0x8ec
github.com/chrislusf/seaweedfs/weed/filesys.(*ContinuousDirtyPages).saveExistingPagesToStorage(0xc0000aca80, 0x10d7ba0, 0xc0003fcc00, 0x0, 0x0, 0x0)
	/home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/filesys/dirty_page.go:107 +0x6c
github.com/chrislusf/seaweedfs/weed/filesys.(*ContinuousDirtyPages).AddPage(0xc0000aca80, 0x10d7ba0, 0xc0003fcc00, 0x278460000, 0xc011966050, 0x20000, 0x20fb0, 0x6fc23ac00, 0x4a817c800, 0x0, ...)
	/home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/filesys/dirty_page.go:70 +0x8f
github.com/chrislusf/seaweedfs/weed/filesys.(*FileHandle).Write(0xc000548410, 0x10d7ba0, 0xc0003fcc00, 0xc00014be30, 0xc011946af8, 0x47fa01, 0x0)
	/home/travis/gopath/src/github.com/chrislusf/seaweedfs/weed/filesys/filehandle.go:141 +0x245
bazil.org/fuse/fs.(*Server).handleRequest(0xc0002cc0c0, 0x10d7ba0, 0xc0003fcc00, 0x10cb020, 0xc000394140, 0xc0000acac0, 0x10d3e60, 0xc00014be30, 0xc00052fef8, 0x10ca6a0, ...)
	/home/travis/gopath/src/bazil.org/fuse/fs/serve.go:1265 +0x1599
bazil.org/fuse/fs.(*Server).serve(0xc0002cc0c0, 0x10d3e60, 0xc00014be30)
	/home/travis/gopath/src/bazil.org/fuse/fs/serve.go:878 +0x410
bazil.org/fuse/fs.(*Server).Serve.func1(0xc0002cc0c0, 0x10d3e60, 0xc00014be30)
	/home/travis/gopath/src/bazil.org/fuse/fs/serve.go:425 +0x6e
created by bazil.org/fuse/fs.(*Server).Serve
	/home/travis/gopath/src/bazil.org/fuse/fs/serve.go:423 +0x321

weed/filesys/dirty_page.go

@@ -139,7 +139,7 @@ func (pages *ContinuousDirtyPages) saveToStorage(ctx context.Context, buf []byte
 	}
 
 	fileUrl := fmt.Sprintf("http://%s/%s", host, fileId)
-	bufReader := bytes.NewReader(pages.Data[:pages.Size])
+	bufReader := bytes.NewReader(buf)
 	uploadResult, err := operation.Upload(fileUrl, pages.f.Name, bufReader, false, "application/octet-stream", nil, "")
 	if err != nil {
 		glog.V(0).Infof("upload data %v to %s: %v", pages.f.Name, fileUrl, err)