Fix signed saturating math functions

[PlasmaPower]

In addition to the test cases, I ran each of the fuzzers for 2 minutes:

for f in FuzzSaturatingAddInt8 FuzzSaturatingAddInt16 FuzzSaturatingAddInt32 FuzzSaturatingAddInt64 FuzzSaturatingSub FuzzSaturatingMulInt8 FuzzSaturatingMulInt16 FuzzSaturatingMulInt32 FuzzSaturatingMulInt64 FuzzSaturatingNegInt8 FuzzSaturatingNegInt16 FuzzSaturatingNegInt32 FuzzSaturatingNegInt64; do go test ./util/arbmath -fuzz "$f" -fuzztime 2m || break; done

I'll run them for longer overnight to make sure but I'm pretty confident in it given this testing.

[tsahee]

LGTM