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.


mysql-test/r/range.result:
  Fix for bug #20732: Partial index and long sjis search with '>' fails sometimes
    - test result.
mysql-test/t/range.test:
  Fix for bug #20732: Partial index and long sjis search with '>' fails sometimes
    - test case.
sql/opt_range.cc:
  Fix for bug #20732: Partial index and long sjis search with '>' fails sometimes
    - check if we have a partial key segment for a Item_func::GT_FUNC;
      if so, don't set NEAR_MIN flag in order to use HA_READ_KEY_OR_NEXT
      instead of HA_READ_AFTER_KEY.
sql/opt_range.h:
  Fix for bug #20732: Partial index and long sjis search with '>' fails sometimes
    - key segment 'flag' slot added.
sql/sql_select.cc:
  Fix for bug #20732: Partial index and long sjis search with '>' fails sometimes
    - test (HA_PART_KEY_SEG | HA_NULL_PART) as we split it in the sql/table.cc
sql/table.cc:
  Fix for bug #20732: Partial index and long sjis search with '>' fails sometimes
    - set HA_NULL_PART flag instead of HA_PART_KEY_SEG in order not to mix them.