avcodec/utils: Use 64bit for aspect ratio calculation in avcodec_string()

Fixes integer overflow
Fixes: 3a45b2ae02f2cf12b7bd99543cdcdae5/asan_heap-oob_1dff502_8022_899f75e1e81046ebd7b6c2394a1419f4.mov

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