matroska: Fix leaking memory allocated for laces.

During error conditions matroska_parse_block may exit without
freeing the memory allocated for laces.

Found via valgrind: http://pastebin.com/E54k8QFU

Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
