avfilter/estdif: avoid signed overflow in slice boundary calculation

deinterlace_slice() computed per-thread row boundaries with int
multiplication height * (jobnr + 1). With a tall frame and many filter
threads the product overflows signed int before the division by nb_jobs.

Use int64_t for the intermediate product before converting back to int
row indices.

Found-by: Kery (Qi Kery <qikeyu2001@outlook.com>)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 200e0cba677b0d478576081ecf12af92b7a4a2d2)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>