avfilter: fix plane validity checks

Fixes out of array accesses

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
static int get_height(const FieldMatchContext *fm, const AVFrame *f, int plane)
{
    return plane ? FF_CEIL_RSHIFT(f->height, fm->vsub) : f->height;
}

static int get_height(const FieldMatchContext *fm, const AVFrame *f, int plane)
{
    return plane ? FF_CEIL_RSHIFT(f->height, fm->vsub) : f->height;
}
