The Item_date_typecast::val_int function doesn't reset null_value flag.
This makes all values that follows the first null value to be treated as nulls
and led to a wrong result.

Now the Item_date_typecast::val_int function correctly sets the null_value flag
for both null and non-null values.


mysql-test/t/cast.test:
  Added a test case for the bug#29898:  Item_date_typecast::val_int doesn't reset
  the null_value flag.
mysql-test/r/cast.result:
  Added a test case for the bug#29898:  Item_date_typecast::val_int doesn't reset
  the null_value flag.
sql/item_timefunc.cc:
  Bug#29898: Item_date_typecast::val_int doesn't reset the null_value flag.
  Now the Item_date_typecast::val_int function correctly sets the null_value flag
  for both null and non-null values.