uninitialized variable used as subscript

Grouping select from a "constant" InnoDB table (a table
of a single row) joined with other tables caused a crash.


mysql-test/r/innodb_mysql.result:
  Added test case for bug bug #44886.
mysql-test/t/innodb_mysql.test:
  Added test case for bug bug #44886.
sql/sql_select.cc:
  Bug #44886: SIGSEGV in test_if_skip_sort_order() -
              uninitialized variable used as subscript
  
  1. The test_if_order_by_key function returned unitialized
     used_key_parts parameter in case of a "constant" InnoDB
     table. Calling function uses this parameter values as
     an array index, thus sometimes it caused a crash.
     The test_if_order_by_key function has been modified
     to set used_key_parts to 0 (no need for ordering).
  
  2. The test_if_skip_sort_order function has been
     modified to accept zero used_key_parts value and
     to prevent an array access by negative index.