compute the skip offset correctly when upsampling
compute the skip offset correctly when upsampling

fixes segfaults with quite some Flash movies.
And it wasn't ALSA's fault, I should stop blaming anything on ALSA...

mltframework/swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,mltframework/swfdec