Bug#57913 large negative number to string conversion functions crash
String object which is used as result container of the item
has uninitialized 'str_charset' field. This object
might be used later to preform some internal operations
and str_charset field is involved in these operations.
It leads to crash.
The fix is to intialize str_charset in my_decimal2string() func.


mysql-test/r/func_str.result:
  test case
mysql-test/t/func_str.test:
  test case
sql/my_decimal.cc:
  intialize str_charset field for result string
  in my_decimal2string() func.