Casting from DECIMAL to SIGNED INTEGER could hit an assertion failure
because the result had more digits than the metadata of
Item_typecast_signed indicated it would have. This happened because
the decimal value is rounded when it's converted to an integer, and
Item_typecast_signed did not take into account that the rounding might
increase the number of digits in the integer part of the decimal
value. For example when rounding 9.9 to 10.

Fixed by removing the logic that tried to compute a minimal maximum
length for the result of the cast, and instead use the default set by
the parent class, Item_int_func. That default is the maximum width of
a 64-bit integer, which should be safe regardless of the input value.

Change-Id: I1670182c632b475eba9face241bcbee78df51fc8