Casting a pointer to some sort of integer and formatting it into a
string isn't valid.  A pointer derived from this hex string won't work
reliably.  Since ffmpeg doesn't provide a nice API for passing our
pointer, we have to think of a different hack: ffmpeg passes the exact
URL pointer to mpdurl_open(), and we can make this string part of a
struct.  This reduces the problem to casting the string back to the
struct.

This is still a workaround, but this is "sort of portable", unless the
ffmpeg people start messing with the URL pointer (which would be valid
according to the API definition).