rtmp: Don't assume path points to a string of nonzero length

If using the new -rtmp_app and -rtmp_playpath parameters,
one can in many cases set the main url to just rtmp://server/.
If the trailing slash is omitted, path is a string of zero length,
and using path+1 will end up reading uninitialized data.

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