handle the other half of the 'maybe' in maybe_get_stat/4
handle the other half of the 'maybe' in maybe_get_stat/4

The riak_core_stat_calc_proc:maybe_get_stat/4 function was missing a
clause for handling the case with its third argument is not `undefined`,
which is the case whenever a stat is being calculated and this process
is waiting on the result.

Kyorai/riak_core