lavf/md5enc: Use AV_HASH_MAX_SIZE

Also increase the buffer size in write_trailer(), since 128
is not enough anymore

Signed-off-by: James Almer <jamrial@gmail.com>
lavu: Add RIPEMD hashing

Includes RIPEMD-128, RIPEMD-160, RIPEMD-256 and RIPEMD-320

Signed-off-by: James Almer <jamrial@gmail.com>