str_to_date() would sometimes render NULL if %D was used as rule other than last.
since this was due to two pointers getting mixed up in the server, this behaviour
seemed somewhat non-deterministic at SQL level.


mysql-test/r/func_time.result:
  Bug #20987: str_to_date doesn't accept user variable for specification
  
  show we can do the usual str_to_date() conversions without triggering the bug.
mysql-test/t/func_time.test:
  Bug #20987: str_to_date doesn't accept user variable for specification
  
  show we can do the usual str_to_date() conversions without triggering the bug.
sql/item_timefunc.cc:
  Bug #20987: str_to_date doesn't accept user variable for specification
  
  str_to_date() used a wrong pointer in %D conversions which could lead to the
  input being cut off after the token matching %D; if the rule required further
  tokens, the conversion would fail and render NULL.