OVERFLOWS

Before this fix, performance schema statistics,
when only counting events but not timing them,
would report column MIN_TIMER_WAIT as a large number 
instead or 0 (incorrect).

The root cause is that PFS_single_stat is initialized with:
- min = ULONGLONGMAX
- max = 0
which is a special marker to indicate that no timed
statistics have been recorded yet,
but this special marker is not checked when computing 
the MIN_TIMER_WAIT value to display.

This fix changed PFS_stat_row::set() to account
for statistics that are only counted and not timed.