stats: missing dimension in stat_set_simple_counter

stats: missing dimension in stat_set_simple_counter

A simple counter is a two dimensional array by threads and
counter index. 28017 introduced an error missing the first
dimension.

If a vector is updated at the same time as a client reads,
an invalid pointer my result. This will be caught by the
optimistic locking after copying out the data, but if
following a pointer outside of the stat segment then
the stat client would crash. Add suitable boundary checks
for access to stat memory segment.

Fixes: 7d29e320fb2855a1ddb7a6af09078b8ed636de01
Type: fix
Signed-off-by: Ole Troan <ot@cisco.com>
Change-Id: I94f124ec71d98218c4eda5d124ac5594743d93d6