DO NOT MERGE YET: Fix url decode

Scanning through the code coverage I noticed an unexcercised path in the handling of ns_uri_decode errors that made me notice that the current implementation might be to lax. Do you think ns_uri_decode should fail on invalid escapes (as shown in the tests added in b45ba51, test failure is intentional) ?
<a href='#crh-start'></a><a href='#crh-data-%7B%22processed%22%3A%20%5B%22https%3A//github.com/cesanta/net_skeleton/pull/44%23issuecomment-61398197%22%5D%2C%20%22comments%22%3A%20%7B%22General%20Comment%22%3A%20%7B%22html_url%22%3A%20%22https%3A//github.com/cesanta/net_skeleton/pull/44%23issuecomment-61398197%22%2C%20%22comments%22%3A%20%5B%7B%22body%22%3A%20%22Yeah%20I%20think%20we%20should%20fail%20on%20invalid%20escapes.%22%2C%20%22created_at%22%3A%20%222014-11-02T09%3A32%3A43Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/601816%3Fv%3D2%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/cpq%22%7D%7D%5D%2C%20%22title%22%3A%20%22General%20Comment%22%7D%7D%7D'></a>
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- [ ] <a href='#crh-comment-General Comment'></a> <img src='http://www.codereviewhub.com/site/github-remaining.png' height=16 width=60>&nbsp;<b><a href='https://github.com/cesanta/net_skeleton/pull/44#issuecomment-61398197'>General Comment</a></b>
- <a href='https://github.com/cpq'><img border=0 src='https://avatars.githubusercontent.com/u/601816?v=2' height=16 width=16'></a> Yeah I think we should fail on invalid escapes.


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