We miss some records sometimes using RANGE method if we have
partial key segments.
Example:
  Create table t1(a char(2), key(a(1)));
  insert into t1 values ('a'), ('xx');
  select a from t1 where a > 'x';
We call index_read() passing 'x' key and HA_READ_AFTER_KEY flag
in the handler::read_range_first() wich is wrong because we have
a partial key segment for the field and might miss records like 'xx'.

Fix: don't use open segments in such a case.