mkvparser: Avoid rollover/truncation in UnserializeString().
mkvparser: Avoid rollover/truncation in UnserializeString().

Change-Id: I355d00b59ec1b7645ae5b4010e292215b5da3a17

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