Curl_urldecode: don't allow NULL as receiver

For a function that returns a decoded version of a string, it seems
really strange to allow a NULL pointer to get passed in which then
prevents the decoded data from being returned!

This functionality was not documented anywhere either.

If anyone would use it that way, that memory would've been leaked.

Bug: https://github.com/bagder/curl/pull/90
Reported-by: Arvid Norberg