If it is negative, it causes segmentation faults in decode_rle.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit f621749d)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>