avfilter/vf_nnedi: Fix segfault when prescreening is disabled

avfilter/vf_nnedi: Fix segfault when prescreening is disabled

Since c737f6edcef74a64f4d0ebcefa970bd31266d512 prescreening is
nevertheless run because of a wrong check: "if (s->prescreen > 0)".
s->prescreen is an array of two function pointers that is contained in
the context and comparing it with 0 (i.e. NULL) is actually undefined
behaviour, because NULL and s->prescreen do not point to the same
object (NULL after all never points to any object). Nevertheless both
Clang as well as GCC compile this to code that treat s->prescreen > 0 as
true, leading to segfaults, because the code then tries to access the
-1th member of an array.

This commit fixes the check as well as another such check a few lines
below.

(Found via compiler warnings enabled by -pedantic:
"ordered comparison between pointer and zero is an extension".)

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>