When the faststart option for the mov/mp4 muxer is used, the current
position (i.e. the size of the already written data pre-shifting) was
evaluated twice: First in an initialization and then again later,
overwriting the first value without having ever touched it. So remove
the initialization.

Also, the clone of this code in the Sega FILM muxer behaves the same and
has been treated the same.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>