hw/display/qxl-render.c: fix qxl_unpack_chunks() chunk size calculation

hw/display/qxl-render.c: fix qxl_unpack_chunks() chunk size calculation

In case of multiple chunks, code in qxl_unpack_chunks() takes size of the
wrong (next in the chain) chunk, instead of using current chunk size.
This leads to wrong number of bytes being copied, and to crashes if next
chunk size is larger than the current one.

Based on the code by Gao Yong.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1628
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Thomas Huth <thuth@redhat.com>