botan2 -- Side channel during ECC key generation

botan2 developers reports:

A timing side channel during ECC key generation could leak information about the
high bits of the secret scalar. Such information allows an attacker to perform a
brute force attack on the key somewhat more efficiently than they would otherwise.
Found by Ján Jančár using ECTester.
Bug introduced in 1.11.20, fixed in 2.9.0