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

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.

src/shared/Auth/BigNumber.h

@@ -72,6 +72,8 @@ class BigNumber
             return t %= bn;
         }
 
+        bool isZero() const;
+
         BigNumber ModExp(const BigNumber &bn1, const BigNumber &bn2);
         BigNumber Exp(const BigNumber &);