plug-ins: improve psp image reader stability by always using the block/chunk length.

Starting from psp file version 4 the specification recommends to always use the block/chunk length to determine the next part of the image. This way it is possible to skip parts you don't know or don't care about or additions in newer versions. This change makes sure to always do this which fixes reading several images which crashed the plug-in before.

Also only try to read layer data if it is a raster layer.

(cherry picked from commit 15ad9522)