rtmpproto: Avoid using uninitialized memory

If the url ends with .flv, we stripped it but didn't initialize
rt->playpath, doing av_strlcat on an uninitialized buffer.

Signed-off-by: Martin Storsjö <martin@martin.st>
rtmpproto: Send a full, absolute timestamp if it isn't monotonically growing

The normal differential timestamps can't handle negative
differences, thus send a full packet header with an absolute
timestamp in these cases.

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