Avoid a harmless data race in RSA decryption
Avoid a harmless data race in RSA decryption

Both threads called Modular_Reducer::reduce on m, which caused the
significant words result to be written twice in an unsynchronized way.
By calling it once beforehand it is computed and cached and so no
additional writes occur.

Found with helgrind.

randombit/botan,randombit/botan,webmaster128/botan,webmaster128/botan,randombit/botan,randombit/botan,randombit/botan,webmaster128/botan,webmaster128/botan,webmaster128/botan