mkvparser/UnserializeInt: fix sign flip
mkvparser/UnserializeInt: fix sign flip

with -funsigned-char the sign will be flipped, e.g., -128 -> 128, use an
explicit signed char.

additionally drop the misguided range check; this is meant as a data
size limit not one for the big-endian encoded ints

Change-Id: Ia10ef811d16acd09cbcd3d95856d9c460c9b7f16

zofuthan/libwebm,webmproject/libwebm,felipebetancur/libwebm,webmproject/libwebm,zofuthan/libwebm,webmproject/libwebm,felipebetancur/libwebm