Don't crash when the packet length is zero

fuzz.go

@@ -0,0 +1,27 @@
+// go:build gofuzz
+//go:build gofuzz
+// +build gofuzz
+
+package sftp
+
+import (
+	"bytes"
+	"context"
+)
+
+type sinkfuzz struct{}
+
+func (*sinkfuzz) Close() error                { return nil }
+func (*sinkfuzz) Write(p []byte) (int, error) { return len(p), nil }
+
+var devnull = &sinkfuzz{}
+
+// To run: go-fuzz-build && go-fuzz
+func Fuzz(data []byte) int {
+	c, err := NewClientPipe(context.Background(), bytes.NewReader(data), devnull)
+	if err != nil {
+		return 0
+	}
+	c.Close()
+	return 1
+}