- Apply Eric Bergen's patch: in join_read_always_key(), move ha_index_init() call
  to before the late NULLs filtering code.
- Backport function comments from 6.0.


mysql-test/r/null_key.result:
  BUG#34945: "ref_or_null queries that are null_rejecting and have a null value crash mysql"
  - Testcase
mysql-test/t/null_key.test:
  BUG#34945: "ref_or_null queries that are null_rejecting and have a null value crash mysql"
  - Testcase
sql/sql_select.cc:
  BUG#34945: "ref_or_null queries that are null_rejecting and have a null value crash mysql"
  - Apply Eric Bergen's patch: in join_read_always_key(), move ha_index_init() call
    to before the late NULLs filtering code.
  - Backport function comments from 6.0