The bug was as follows: When merge_key_fields() encounters "t.key=X OR t.key=Y" it will 
try to join them into ref_or_null access via "t.key=X OR NULL". In order to make this 
inference it checks if Y<=>NULL, ignoring the fact that value of Y may be not yet known.

The fix is that the check if Y<=>NULL is made only if value of Y is known (i.e. it is a
constant).
TODO: When merging to 5.0, replace used_tables() with const_item() everywhere in merge_key_fields().


mysql-test/r/innodb_mysql.result:
  Testcase for BUG16798
mysql-test/t/innodb_mysql.test:
  Testcase for BUG16798
sql/sql_select.cc:
  BUG#16798: Inapplicable ref_or_null query plan and bad query result on random occasions 
  In merge_key_fields() don't call val->is_null() if the value of val is not known.