message: fix critical when trying to create a message from invalid URI
soup_message_new_from_uri() raises a critical if SOUP_URI_IS_VALID returns FALSE. This macro also checks if the URI host is not an empty string, which was not checked in the caller code.
Fix critical when running such GStreamer pipeline: gst-launch-1.0 souphttpsrc location=http:///not-there.ogg ! fakesink