MONTHNAME(0) claims that it is about to return NOT NULL
value, whereas it actually returns NULL.

As a result storage_engine variable (which cannot be NULL)
protection was bypassed and NULL value was accepted, causing
server crash.

Fixed MONTHNAME(0) to report valid NULL flag.

mysql-test/r/func_time.result:
  A test case for BUG#11766720.
mysql-test/t/func_time.test:
  A test case for BUG#11766720.
sql/item_timefunc.cc:
  MONTHNAME(0) must report NULL, as opposed to base class
  MONTH(0) which is NOT NULL.
  
  Fixed Item_func_monthname to inherit from Item_str_func
  instead of Item_func_month.
sql/item_timefunc.h:
  MONTHNAME(0) must report NULL, as opposed to base class
  MONTH(0) which is NOT NULL.
  
  Fixed Item_func_monthname to inherit from Item_str_func
  instead of Item_func_month.