avformat/mov: ignore samples overflowing next_root_atom

This fixes #2657.
ISML movies produced by Microsoft Expression Encoder 4 seem to have invalid
sample entries in their trun/tfhd for data tracks. As a result, too much bytes
are read for these tracks to the point that next_root_atom can go out of
buffer, which makes the encoding fail if the input is not seekable.

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