The function str_to_date has a field to say whether it's invoked constant 
arguments. But this member was not initialized, causing the function to
think that it could use a cache of the format type when said cache was in 
fact not initialized.
Fixed by initializing the field to false.


mysql-test/r/type_date.result:
  Bug#30942: Test result
mysql-test/t/type_date.test:
  Bug#30942: Test case
sql/item_timefunc.h:
  Bug#30942: Initialized const_item to false