Before MariaDB 10.3.5, the binlog position was stored in the TRX_SYS page,
while after it is stored in rollback segments. There is code to read the
legacy position from TRX_SYS to handle upgrades. The problem was if the
legacy position happens to compare larger than the position found in
rollback segments; in this case, the old TRX_SYS position would incorrectly
be preferred over the newer position from rollback segments.

Fixed by always preferring a position from rollback segments over a legacy
position.
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>