HOUR(), MINUTE(), ... returned spurious results when used on a DATE-cast.
This happened because DATE-cast object did not overload get_time() method
in superclass Item. The default method was inappropriate here and
misinterpreted the data.

Patch adds missing method; get_time() on DATE-casts now returns SQL-NULL
on NULL input, 0 otherwise. This coincides with the way DATE-columns
behave.

Also fixes similar bug in Date-Field now.


mysql-test/r/cast.result:
  Show that HOUR(), MINUTE(), ... return sensible values when used
  on DATE-cast objects, namely NULL for NULL-dates and 0 otherwise.
  Show that this coincides with how DATE-columns behave.
mysql-test/r/type_date.result:
  Show that HOUR(), MINUTE(), ... return sensible values when used
  on DATE-fields.
mysql-test/t/cast.test:
  Show that HOUR(), MINUTE(), ... return sensible values when used
  on DATE-cast objects, namely NULL for NULL-dates and 0 otherwise.
  Show that this coincides with how DATE-columns behave.
mysql-test/t/type_date.test:
  Show that HOUR(), MINUTE(), ... return sensible values when used
  on DATE-fields.
sql/field.cc:
  Add get_time() method to DATE-field object to overload
  the method in Field superclass that would return spurious
  results. Return zero-result.
sql/field.h:
  Add get_time() declaration to date-field class
sql/item_timefunc.cc:
  Add get_time() method to DATE-cast object to overload
  the method in Item superclass that would return spurious
  results. Return zero-result; flag NULL if input was NULL.
sql/item_timefunc.h:
  Add get_time() declaration to DATE-cast object.