avformat_find_stream_info: fallback to timestamps in analyzeduration check in more cases

This fixes speex in rtmp
Fixes Ticket2409

the nellymoser in flv case actually needs larger analyzeduration. The code
previously just failed to calculate the duration

If this causes any problems, like premature analyze/probe end, please report!

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