Fixing Item::decimal_precision() to return at least one digit.
This fixes the problem reported in MDEV.

Also, fixing Item_func_signed::fix_length_and_dec() to reserve
space for at least one digit (plus one character for an optional sign).
This is needed to have CONVERT(expr,SIGNED) and CONVERT(expr,UNSIGNED)
create correct string fields when they appear in string context, e.g.:
  CREATE TABLE t1 AS SELECT CONCAT(CONVERT('',SIGNED));