nassh 0.8.22.4: Fix relayPrefix with explicit proxyHost.

Fix to use the correct relayHost in the event that the
relayPrefixField is in effect, but the prefix had already
been included in the proxyHost.

Change-Id: I0a189cf8e75b47eb60860debe992739ac65de841
Reviewed-on: https://chromium-review.googlesource.com/178429
Reviewed-by: Robert Ginda <rginda@chromium.org>
Tested-by: Robert Ginda <rginda@chromium.org>