device: fix shadowing of err in IpcHandle

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>