raop: Return NULL if IPv6 since we do not support it (yet)

Since we do not support IPv6 yet, do not create a client object
when we encounter a device with IPv6 address.

This patch was originally proposed by josla972
(https://github.com/josla972) and an improvement was proposed by
Peter Fern <github@obfusc8.org>, then reformatted and committed
by Hajime Fujita <crisp.fujita@nifty.com>.

See https://github.com/hfujita/pulseaudio-raop2/pull/17 for the
full conversation.