KaxBlock: don't access beyond enf-of-buffer for one frame in EBML lacing

See https://trac.bunkus.org/ticket/1096 test case
id:000002,sig:06,src:000000,op:flip2,pos:582
version: bump to 1.4.2
cosmetics: unify indentation
KaxBlock::ReadData(): use safe/checked memory access

Unchecked reading from memory locations works for cases in which all the
data is valid, but if it isn't then this leads to invalid memory access
and segmentation faults.

See https://trac.bunkus.org/ticket/1096
cosmetics: re-indent KaxBlock::ReadData() after wrapping in try/catch block