yadif: redesign first and last 2 lines handling.

The previous code dependent on the input buffer matching the
buffer that has been provided by yadifs get_buffer.
The API does in now way gurantee this though its often true.
This fixes some out of array reads.
The regression test checksums change due to "out of picture" values
being initialized differently.
There should be no visual difference in the filters output

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
