avfilter/vf_transpose: Fix rounding error

Fixes out of array access
Fixes: asan_heap-oob_7f875d_3482_cov_1818465256_ssudec.mov

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
