PR# 19636 STRING_32.hash_code may have negative values

When using invalid Unicode characters in STRING_32, the hash code may be negative. For example:

		local
			s: STRING_32
		do
			create s.make (1)
			s.append_character ({NATURAL_32}.max_value.to_character_32)
			print (s.hash_code)

prints -1. But the postcondition of `hash_code`says:

		good_hash_value: Result >= 0

In the Gobo libraries, the following pattern is used:

		Result := some_value_which_may_be_nagetive
		if Result < 0 then
			Result := -(Result + 1)
		end

--
Eric Bezault