check a for 0 in Legendre

libi2pd/Elligator.cpp

@@ -160,7 +160,8 @@ namespace crypto
 	
 	int Elligator2::Legendre (const BIGNUM * a, BN_CTX * ctx) const
 	{
-		// assume a < p, so don't check for a % p = 0
+		// assume a < p, so don't check for a % p = 0, but a = 0 only
+		if (BN_is_zero(a)) return 0; 
 		BIGNUM * r = BN_CTX_get (ctx);
 		BN_mod_exp (r, a, p12, p, ctx); // 	r = a^((p-1)/2) mod p
 		if (BN_is_word(r, 1))