sql: fix usage of not initialized index_stat

Query planner uses a temporary index definition object 'to
represent the primary key index'. Whatever real purpose of this
index_def is (query planner wasn't changed since SQLite merge,
and may be broken), its opts.stat field pointed at a partially
initialized index_stat structure. Which is supposed to be used by
the planner to make decisions such as search by which index would
be the optimal.

The patch initializes the statistics with 0.

Part of #4609