BUG/MINOR: threads: fix multiple use of argument inside HA_ATOMIC_UPDATE_{MIN,MAX}()

BUG/MINOR: threads: fix multiple use of argument inside HA_ATOMIC_UPDATE_{MIN,MAX}()

Just like in previous patch, it happens that HA_ATOMIC_UPDATE_MIN() and
HA_ATOMIC_UPDATE_MAX() would evaluate the (val) argument up to 3 times.
However this time it affects both thread and non-thread versions. It's
strange because the copy was properly performed for the (new) argument
in order to avoid this. Anyway it was done for the "val" one as well.

A quick code inspection showed that this currently has no effect as
these macros are fairly limited in usage.

It would be best to backport this for long-term stability (till 1.8)
but it will not fix an existing bug.