avisynth: Cosmetics

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit f277d6bf42329d481f58b6147f6dc4130f198ba5)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avisynth: Exit gracefully when trying to serve video from v2.5.8.

'Fixes' ticket #2526 insofar as it stops 2.5.8 from crashing and
tells the user to upgrade to 2.6 if they want to make video input
work. A real solution to #2526 would be to get video input from
2.5.8 to work right.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 9db353bc4727d2a184778c110cf4ea0b9d1616cb)

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