vct: avoid read descriptors past the buffer

vct: avoid read descriptors past the buffer

On some ATSC streams found in KR, the descriptor_length at the
VCT tables are found to be bigger than the remaining buffer
size.

While this could be due to some other problem, add a logic to
prevent going paste the buffer.

Likely, this patch should also be ported to other places where
dvb_parse_descriptors() is called.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>