mov: fix DTS calculation for samples with negative stts duration

A negative sample duration is invalid according to the spec, but there
are samples that use it for the DTS calculation, e.g.:
http://files.1f0.de/samples/mp4-negative-stts-problem.mp4

These currently get out of A/V sync.

Also change the logging type to AV_LOG_WARNING, because decoding the
sample can continue.

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
