rtmpproto: Don't mistake app for playpath.

For URLs "rtmp://server[:port]/foo

" determine what `foo` refers to. If
application name has been defined by the user assume that `foo` is a
playpath, otherwise assume application name.

Signed-off-by: Kacper Michajłow <kasper93@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>