The TABLE::reginfo.impossible_range is used by the optimizer to indicate
that the condition applied to the table is impossible. It wasn't initialized
at table opening and this might lead to an empty result on complex queries:
a query might set the impossible_range flag on a table and when the query finishes,
all tables are returned back to the table cache. The next query that uses the table
with the impossible_range flag set and an index over the table will see the flag
and thus return an empty result.

The open_table function now initializes the TABLE::reginfo.impossible_range
variable.