This works around a libzvbi bug (a corner case in the teletext spec):

https://sourceforge.net/p/zapping/bugs/203/
https://sourceforge.net/p/zapping/patches/20/

Fixes samples/ffmpeg-bugs/trac/ticket2086/RBT_20100801_1835.ts.
Signed-off-by: Marton Balint <cus@passwd.hu>