Fixes integer overflow
Fixes: 1e32c6c591d940337c20b197ec1c4d3d/asan_heap-oob_4a52e5_8946_0bb0d9e863def56005e49f1d89bdc94d.avi

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>