avcodec/msrle: use av_image_get_linesize() to calculate the linesize

      Fixes out of array access
Fixes: 14a74a0a2dc67ede543f0e35d834fbbe-asan_heap-oob_49572c_556_cov_215466444_44_001_engine_room.mov
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>