avidec: Fix regression with chunks that are larger than the file.

This commit makes the check specific to the case that needs it.

Regression was introduced by
commit 62adc60b97d854507d07a21b2f370ab5c69e6b7b
Author: Michael Niedermayer <michaelni@gmx.at>
Date:   Fri Dec 16 06:13:04 2011 +0100

    avidec: Check that the header chunks fit in the available filesize.
    Fixes Ticket771
    Bug found by: Diana Elena Muscalu

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

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