fix flv new stream handling after head already parsed

fix flv new stream handling after head already parsed

This fixes stream lookup in flv demuxer. When used with librtmp
protocol streams are sometimes added after head arrived. If the
first stream added in flv header reader is Audio stream then it
is messed with Video stream added later in the code patched.
The result is I have 2 Audio streams (first of them is said to have
a video codec like h264) instead of Audio/Video pair.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>