[8445] Fixed redundant calculation of v and s on every login Author: arrai

The SRP-6 specifications clearly say, that v and s are only to be
    calculated on registering a user and changing his password; calculating
    them on every login is plain waste.

--HG--
branch : trunk

src/shared/Auth/BigNumber.h

@@ -74,6 +74,8 @@ class BigNumber
             return t %= bn;
         }
 
+        bool isZero() const;
+
         BigNumber ModExp(const BigNumber &bn1, const BigNumber &bn2);
         BigNumber Exp(const BigNumber &);