[INTERNAL] ensure that an URL have a host part

The chat currently allows the creation of hyperlinks for
URL without a host part. As those hyperlinks would point to
"nothing" make sure that such URLs are not transformed to hyperlinks.

Change-Id: Ie42e7e95d16ee85e407e1087e9ff00948fa11c02
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/980
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>