HID: ft260: fix a NULL pointer dereference in ft260_i2c_write

The zero-length passed into the ft260_i2c_write() triggered the
NULL pointer dereference in the debug message on data[0] access.
Since the controller does not support a write of zero length,
let's not allow it.

Before:

$ sudo i2ctransfer -y 13 w0@0x51
Killed

After:

$ sudo i2ctransfer -y 13 w0@0x51
Error: Sending messages failed: Invalid argument
Reported-by: Enrik Berkhan <Enrik.Berkhan@inka.de>
Signed-off-by: Michael Zaidman <michael.zaidman@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-ft260.c

@@ -409,6 +409,9 @@ static int ft260_i2c_write(struct ft260_device *dev, u8 addr, u8 *data,
 	struct ft260_i2c_write_request_report *rep =
 		(struct ft260_i2c_write_request_report *)dev->write_buf;
 
+	if (len < 1)
+		return -EINVAL;
+
 	rep->flag = FT260_FLAG_START;
 
 	do {