Check the teletext framing code in PES buffer for each of the received lines
and only copy those with correct framing code (p_block->p_buffer[3]) to the
p_sliced buffer that is later forwarded to ZVBI vbi_decode. Invalid lines
will not reach vbi_decode anymore.

When also packets with erroneous framing code are sent to vbi_decode (often
0x00 with some noise), in most cases those are decoded as packet 1/2,
causing the second text line on the teletext page to be overwritten with
spaces. So we need to avoid sending such packets with invalid framing code
to vbi_decode.

Close #14191
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>