Use all 32 bits of the timestamp when calculating flv duration.

At the moment, duration is mainly set from the metadata packet. If that is not
available, the fallback is checking the low 24 bits of the last packet. This is
not enough for files over 4,6 hours in length, so read all 32 bits instead.
patch by Martin Storsjö, martin martin st

Originally committed as revision 19791 to svn://svn.ffmpeg.org/ffmpeg/trunk