mpegvideo: Avoid 32-bit wrapping of linesize multiplications

This makes sure that linesize * start_y doesn't overflow, so that
emulated_edge_mc can get back the original value if needed.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
