rtsp: Don't pass string pointer as format string to ff_url_join

rtsp: Don't pass string pointer as format string to ff_url_join

In this case, the string that was passed couldn't contain
user-defined data and thus there was no risk for injection
bugs, but it's safer this way, if we later change the
content of the options string.

Signed-off-by: Martin Storsjö <martin@martin.st>