libavformat/fifo: Fix initialization of underlying AVFormatContext

Muxers may want to directly access filename in stored in
AVFormatContext. For example in case of RTSP, the filename (url)
is used by the muxer to extract parameters of the connection.
These muxers will fail when used with fifo pseudo-muxer.

This commit fixes this issue by passing filename from AVFormatContext
of fifo pseudo-muxer to all AVFormatContext(s) of underlying muxers
during initialization.

Signed-off-by: Jan Sebechlebsky <sebechlebskyjan@gmail.com>