device: fix shadowing of err in IpcHandle

The declaration of err in

	nextByte, err := buffered.ReadByte

shadows the declaration of err in

	op, err := buffered.ReadString('\n')

above. As a result, the assignments to err in

	err = ipcErrorf(ipc.IpcErrorInvalid, "trailing character in UAPI get: %c", nextByte)

and in

	err = device.IpcGetOperation(buffered.Writer)

do not modify the correct err variable.

Found by staticcheck.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

device/uapi.go

@@ -403,7 +403,8 @@ func (device *Device) IpcHandle(socket net.Conn) {
 		case "set=1\n":
 			err = device.IpcSetOperation(buffered.Reader)
 		case "get=1\n":
-			nextByte, err := buffered.ReadByte()
+			var nextByte byte
+			nextByte, err = buffered.ReadByte()
 			if err != nil {
 				return
 			}