being used without being def

Inside method Item_func_unsigned::val_int, the variable value 
can be returned without being initialized when the CAST argument
is of type DECIMAL and has a NULL value. This gives a run-time 
error when building debug binaries using Visual C++ 2005.

Solution: Initialize value to 0


mysql-test/t/cast.test:
  bug#28250: There is no need for an extra test case, but we
  recognize that this one catches the bug.
sql/item_func.cc:
  bug#28250: initialization of value.