keyring: Avoid undefined out-of-range shift

Detected with UndefinedBehaviourSanitizer, which will warn on about 50% of calls to this function, when s[3] is 128 or more, because id is signed, so 128 << 24 is undefined signed overflow.

All we want here is a random non-negative signed int (in the range 0 to 2**31-1, with 31 bits varying). The intention seemed to be to generate a random unsigned int, cast it to signed, and then negate it if negative, but it seems simpler and more obviously correct to just make sure the most significant byte fits in the non-negative range.