lib/url: fix no-scheme URL parsing

We were incorrectly dropping the first strlen("file://") characters from
URLs with no scheme:

--- test/urls/data/localpath.test	2013-05-02 17:26:48.826359036 +0800
+++ /tmp/tmp.gn4JsWLw5o	2013-05-02 17:26:50.262364613 +0800
@@ -2,6 +2,6 @@
 scheme	file
 host	(null)
 port	(null)
-path	/test/path/to/local/file
-dir	/test/path/to/local/
+path	ath/to/local/file
+dir	ath/to/local/
 file	file

This change fixes the issue by indicating "no scheme found" by a NULL
return from pb_url_find_scheme, and hadling it appropriately. We add a
testcase too.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>