fuse: ignore unrecognized opcodes

Go-FUSE has been trailing the kernel, and the kernel (usually) is
careful to only send opcodes that are recognized. However, on GKE
(GCP) the undefined opcode 2016 has been seen, leading to a crash.

Fixes #276.

fuse/request.go

@@ -217,8 +217,14 @@ func (r *request) outData() unsafe.Pointer {
 // serializeHeader serializes the response header. The header points
 // to an internal buffer of the receiver.
 func (r *request) serializeHeader(flatDataSize int) (header []byte) {
-	dataLength := r.handler.OutputSize
+	var dataLength uintptr
+
+	if r.handler != nil {
+		dataLength = r.handler.OutputSize
+	}
 	if r.status > OK {
+		// only do this for positive status; negative status
+		// is used for notification.
 		dataLength = 0
 	}