Stack Buffer Overflow via `gmp_sprintf`in `BLSSignature` and `BLSSigShare`

Summary

A security researcher identified a stack buffer overflow vulnerability in libBLS. While the risk was very low and nothing directly exploitable through the Network or Adjacent Network (the vulnerability requires local access to a machine and subsequent code changes to libBLS), the security team went ahead and made a quick change to mitigate this behavior. Because a change was made as a result of this report, the security team thanks @voiddy and awarded a bounty.